<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2024 Workiva-->
<!--r:4eacbfee-b7f7-4d2c-b4bb-5620d33fa167,g:4e7703e4-2f16-4cb8-afb0-b74b11bb0213-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.grail.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="simple" xlink:href="gral-20240630.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETS"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_87b8e443-0c49-4a05-9de2-8b388244ed59" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_14731980-7cbd-49e9-90d7-29422753beb3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_87b8e443-0c49-4a05-9de2-8b388244ed59" xlink:to="loc_us-gaap_StatementTable_14731980-7cbd-49e9-90d7-29422753beb3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b41689cd-18bc-4b20-9a54-79e4109852b2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_14731980-7cbd-49e9-90d7-29422753beb3" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b41689cd-18bc-4b20-9a54-79e4109852b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_b41689cd-18bc-4b20-9a54-79e4109852b2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b41689cd-18bc-4b20-9a54-79e4109852b2" xlink:to="loc_us-gaap_RelatedPartyDomain_b41689cd-18bc-4b20-9a54-79e4109852b2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_8755f1aa-07ba-4867-bea1-29ecf17f7d09" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b41689cd-18bc-4b20-9a54-79e4109852b2" xlink:to="loc_us-gaap_RelatedPartyDomain_8755f1aa-07ba-4867-bea1-29ecf17f7d09" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_1e610f0f-902f-48f5-8dd9-7d7979fe2bb3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_8755f1aa-07ba-4867-bea1-29ecf17f7d09" xlink:to="loc_us-gaap_RelatedPartyMember_1e610f0f-902f-48f5-8dd9-7d7979fe2bb3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_786dccf4-5f74-4adb-b282-f09b709df66c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_8755f1aa-07ba-4867-bea1-29ecf17f7d09" xlink:to="loc_us-gaap_NonrelatedPartyMember_786dccf4-5f74-4adb-b282-f09b709df66c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_87b8e443-0c49-4a05-9de2-8b388244ed59" xlink:to="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_de8747e8-3b50-46ee-b064-7490899c44a7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:to="loc_us-gaap_AssetsCurrentAbstract_de8747e8-3b50-46ee-b064-7490899c44a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_6d3052c7-2ba8-4658-b70f-a0056fd0a903" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_de8747e8-3b50-46ee-b064-7490899c44a7" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_6d3052c7-2ba8-4658-b70f-a0056fd0a903" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_5e329bc4-1234-4668-9944-aa6fa79c3211" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_de8747e8-3b50-46ee-b064-7490899c44a7" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_5e329bc4-1234-4668-9944-aa6fa79c3211" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Supplies_15e1ed9d-d7d0-414a-9bc0-edc80c43773c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Supplies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_de8747e8-3b50-46ee-b064-7490899c44a7" xlink:to="loc_us-gaap_Supplies_15e1ed9d-d7d0-414a-9bc0-edc80c43773c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseCurrent_abda50c9-d34d-4150-b3fe-f92fe8855a87" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PrepaidExpenseCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_de8747e8-3b50-46ee-b064-7490899c44a7" xlink:to="loc_us-gaap_PrepaidExpenseCurrent_abda50c9-d34d-4150-b3fe-f92fe8855a87" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_5e2f3daf-7cee-40d3-b3c8-b075d5a6d2ad" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_de8747e8-3b50-46ee-b064-7490899c44a7" xlink:to="loc_us-gaap_AssetsCurrent_5e2f3daf-7cee-40d3-b3c8-b075d5a6d2ad" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_7f831f44-b8b1-45cd-bbde-49ef19bad0ce" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_7f831f44-b8b1-45cd-bbde-49ef19bad0ce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_576aaabd-a9da-4386-b5e0-afb11004975e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_576aaabd-a9da-4386-b5e0-afb11004975e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashNoncurrent_4e32cf36-e157-4018-a002-4b946f1a88cf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedCashNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:to="loc_us-gaap_RestrictedCashNoncurrent_4e32cf36-e157-4018-a002-4b946f1a88cf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_4a039fd0-d6cd-45c2-962e-794125e2603a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_4a039fd0-d6cd-45c2-962e-794125e2603a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_472efb12-88b0-4f30-9858-4b6c2f7ab2c6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:to="loc_us-gaap_Goodwill_472efb12-88b0-4f30-9858-4b6c2f7ab2c6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_1c525db2-0e81-4b48-8851-bf379c58c168" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_1c525db2-0e81-4b48-8851-bf379c58c168" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_ab22b9ba-a938-4041-b43a-45fa178154d7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_aa343c49-dda3-4318-9159-f5274f570565" xlink:to="loc_us-gaap_Assets_ab22b9ba-a938-4041-b43a-45fa178154d7" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_87b8e443-0c49-4a05-9de2-8b388244ed59" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_fc720c0c-c147-4b01-a401-e10864703496" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_fc720c0c-c147-4b01-a401-e10864703496" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_d98a258b-35db-43ea-b77a-79a418aa77be" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_fc720c0c-c147-4b01-a401-e10864703496" xlink:to="loc_us-gaap_AccountsPayableCurrent_d98a258b-35db-43ea-b77a-79a418aa77be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_6c17bbf3-5b05-43fd-a256-10a0d3f6ab61" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_fc720c0c-c147-4b01-a401-e10864703496" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_6c17bbf3-5b05-43fd-a256-10a0d3f6ab61" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCompensationLiabilityCurrent_d8ea834e-41cb-46be-8b25-fb7005d07c5b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredCompensationLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_fc720c0c-c147-4b01-a401-e10864703496" xlink:to="loc_us-gaap_DeferredCompensationLiabilityCurrent_d8ea834e-41cb-46be-8b25-fb7005d07c5b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_6118085f-6f60-428e-a78f-de0cf0e14f28" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_fc720c0c-c147-4b01-a401-e10864703496" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_6118085f-6f60-428e-a78f-de0cf0e14f28" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesCurrent_525b4342-44b7-4ffc-ab85-1d99e0fca976" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_fc720c0c-c147-4b01-a401-e10864703496" xlink:to="loc_us-gaap_OtherLiabilitiesCurrent_525b4342-44b7-4ffc-ab85-1d99e0fca976" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_7f534403-eb6d-4d89-9c64-59ca51768c0b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_fc720c0c-c147-4b01-a401-e10864703496" xlink:to="loc_us-gaap_LiabilitiesCurrent_7f534403-eb6d-4d89-9c64-59ca51768c0b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_25cb6424-ad33-4241-98c5-014e42a0390c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_25cb6424-ad33-4241-98c5-014e42a0390c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_97984ada-ec72-4169-9400-7a26df55f764" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_97984ada-ec72-4169-9400-7a26df55f764" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_216669f0-3deb-47b9-9782-badeee2b31f7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_216669f0-3deb-47b9-9782-badeee2b31f7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_3a727700-4c05-4a5e-ae3e-bacc8dcd587d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_Liabilities_3a727700-4c05-4a5e-ae3e-bacc8dcd587d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_10c63bbd-d923-4d17-84e8-4bad2010ebc2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PreferredStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_PreferredStockValue_10c63bbd-d923-4d17-84e8-4bad2010ebc2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_2d232c99-f4a4-47a6-bfc6-b47ffff752e8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_CommonStockValue_2d232c99-f4a4-47a6-bfc6-b47ffff752e8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapital_6dd59113-6de3-4d70-8bb5-f6381629f595" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdditionalPaidInCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_AdditionalPaidInCapital_6dd59113-6de3-4d70-8bb5-f6381629f595" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MembersCapital_3729e05a-dd05-420c-bec5-1d04cb663f1f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MembersCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_MembersCapital_3729e05a-dd05-420c-bec5-1d04cb663f1f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_0d819db6-b7fa-4242-bc57-74b21c98d2b9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_0d819db6-b7fa-4242-bc57-74b21c98d2b9" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_bfbf3578-eb8c-40af-bd2b-73f9a3b42c99" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_bfbf3578-eb8c-40af-bd2b-73f9a3b42c99" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_f071621a-72ee-4fd7-971d-f8a0ad8f23d4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_StockholdersEquity_f071621a-72ee-4fd7-971d-f8a0ad8f23d4" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MembersEquity_85ac25f4-8c64-4cfe-be53-0730758d2a35" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MembersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_MembersEquity_85ac25f4-8c64-4cfe-be53-0730758d2a35" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_fe8a1a60-c0f8-44da-896f-57f1fc6523a1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_7cd1e807-0028-4e69-bf29-90eef460c150" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_fe8a1a60-c0f8-44da-896f-57f1fc6523a1" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="simple" xlink:href="gral-20240630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_f8678344-1221-458e-bd8a-6288c1bd5e5c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:to="loc_us-gaap_StatementTable_f8678344-1221-458e-bd8a-6288c1bd5e5c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_c3acaf44-3cc1-4aff-90e3-487c998c48da" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_f8678344-1221-458e-bd8a-6288c1bd5e5c" xlink:to="loc_srt_ProductOrServiceAxis_c3acaf44-3cc1-4aff-90e3-487c998c48da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_c3acaf44-3cc1-4aff-90e3-487c998c48da_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_c3acaf44-3cc1-4aff-90e3-487c998c48da" xlink:to="loc_srt_ProductsAndServicesDomain_c3acaf44-3cc1-4aff-90e3-487c998c48da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_3bd3a1b7-54f8-40ac-b8c5-10901c27a5a1" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_c3acaf44-3cc1-4aff-90e3-487c998c48da" xlink:to="loc_srt_ProductsAndServicesDomain_3bd3a1b7-54f8-40ac-b8c5-10901c27a5a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ScreeningRevenueMember_9ca27af8-83f6-48f0-9a6c-51e22d1f6d6e" xlink:href="gral-20240630.xsd#gral_ScreeningRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_3bd3a1b7-54f8-40ac-b8c5-10901c27a5a1" xlink:to="loc_gral_ScreeningRevenueMember_9ca27af8-83f6-48f0-9a6c-51e22d1f6d6e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_DevelopmentalServicesRevenueMember_b136152f-1784-4015-b7d3-1327e99e3ed1" xlink:href="gral-20240630.xsd#gral_DevelopmentalServicesRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_3bd3a1b7-54f8-40ac-b8c5-10901c27a5a1" xlink:to="loc_gral_DevelopmentalServicesRevenueMember_b136152f-1784-4015-b7d3-1327e99e3ed1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b15858b1-f547-4ac8-b86e-62d9729cb052" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_f8678344-1221-458e-bd8a-6288c1bd5e5c" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b15858b1-f547-4ac8-b86e-62d9729cb052" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_b15858b1-f547-4ac8-b86e-62d9729cb052_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b15858b1-f547-4ac8-b86e-62d9729cb052" xlink:to="loc_us-gaap_RelatedPartyDomain_b15858b1-f547-4ac8-b86e-62d9729cb052_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_7c0164d5-5f8f-47fd-ae56-9066dcb0e9f2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_b15858b1-f547-4ac8-b86e-62d9729cb052" xlink:to="loc_us-gaap_RelatedPartyDomain_7c0164d5-5f8f-47fd-ae56-9066dcb0e9f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_e70fd27e-ed71-409f-8770-3238dde64836" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_7c0164d5-5f8f-47fd-ae56-9066dcb0e9f2" xlink:to="loc_us-gaap_RelatedPartyMember_e70fd27e-ed71-409f-8770-3238dde64836" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_86dd0184-04ab-422c-a8ca-fb75fcb175e8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_7c0164d5-5f8f-47fd-ae56-9066dcb0e9f2" xlink:to="loc_us-gaap_NonrelatedPartyMember_86dd0184-04ab-422c-a8ca-fb75fcb175e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerAbstract_600b2d20-06a9-4a2f-80f3-7606d30ef71c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerAbstract_600b2d20-06a9-4a2f-80f3-7606d30ef71c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_d04bd581-a225-437c-83ab-62c8a2ddef7e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueFromContractWithCustomerAbstract_600b2d20-06a9-4a2f-80f3-7606d30ef71c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_d04bd581-a225-437c-83ab-62c8a2ddef7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingCostsAndExpensesAbstract_9f840770-5b10-4c63-b31e-c3a50b4fc738" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingCostsAndExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:to="loc_us-gaap_OperatingCostsAndExpensesAbstract_9f840770-5b10-4c63-b31e-c3a50b4fc738" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_774db852-d64b-4062-b3f3-3bba2db69931" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingCostsAndExpensesAbstract_9f840770-5b10-4c63-b31e-c3a50b4fc738" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_774db852-d64b-4062-b3f3-3bba2db69931" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSoldAmortization_aa335dfe-e0bc-4a29-b20e-f91e2a571ef5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfGoodsAndServicesSoldAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingCostsAndExpensesAbstract_9f840770-5b10-4c63-b31e-c3a50b4fc738" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSoldAmortization_aa335dfe-e0bc-4a29-b20e-f91e2a571ef5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_08adb5c6-7ea4-4fd3-b5c0-f6c08964df50" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingCostsAndExpensesAbstract_9f840770-5b10-4c63-b31e-c3a50b4fc738" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_08adb5c6-7ea4-4fd3-b5c0-f6c08964df50" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpense_ef83cc24-2309-4298-b883-e2bb4d60411c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SellingAndMarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingCostsAndExpensesAbstract_9f840770-5b10-4c63-b31e-c3a50b4fc738" xlink:to="loc_us-gaap_SellingAndMarketingExpense_ef83cc24-2309-4298-b883-e2bb4d60411c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_081107ac-9ed3-40fb-a873-a1d04151a793" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingCostsAndExpensesAbstract_9f840770-5b10-4c63-b31e-c3a50b4fc738" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_081107ac-9ed3-40fb-a873-a1d04151a793" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAndIntangibleAssetImpairment_179786a9-96f0-4a52-b52f-742adf980497" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillAndIntangibleAssetImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingCostsAndExpensesAbstract_9f840770-5b10-4c63-b31e-c3a50b4fc738" xlink:to="loc_us-gaap_GoodwillAndIntangibleAssetImpairment_179786a9-96f0-4a52-b52f-742adf980497" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpenses_3b9f06b3-0143-498b-920b-e604635d3b02" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingCostsAndExpensesAbstract_9f840770-5b10-4c63-b31e-c3a50b4fc738" xlink:to="loc_us-gaap_CostsAndExpenses_3b9f06b3-0143-498b-920b-e604635d3b02" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_9f765c55-3ca6-4e20-975c-d1d4db1727a5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:to="loc_us-gaap_OperatingIncomeLoss_9f765c55-3ca6-4e20-975c-d1d4db1727a5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIncomeAndExpensesAbstract_7192d9f6-1abe-4bd0-8a9d-c9962dc929ac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherIncomeAndExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:to="loc_us-gaap_OtherIncomeAndExpensesAbstract_7192d9f6-1abe-4bd0-8a9d-c9962dc929ac" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterest_892d38c6-f595-4319-93a1-e2981bba77ec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InvestmentIncomeInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherIncomeAndExpensesAbstract_7192d9f6-1abe-4bd0-8a9d-c9962dc929ac" xlink:to="loc_us-gaap_InvestmentIncomeInterest_892d38c6-f595-4319-93a1-e2981bba77ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_bb3ab55e-338f-421e-97eb-b5520f0b1672" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherIncomeAndExpensesAbstract_7192d9f6-1abe-4bd0-8a9d-c9962dc929ac" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_bb3ab55e-338f-421e-97eb-b5520f0b1672" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_c49a1bad-390c-4824-a878-d10765ca162b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherIncomeAndExpensesAbstract_7192d9f6-1abe-4bd0-8a9d-c9962dc929ac" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_c49a1bad-390c-4824-a878-d10765ca162b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_26780d2b-d571-4df1-ac5b-aa7de31e25b9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_26780d2b-d571-4df1-ac5b-aa7de31e25b9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_5e7279d3-a41f-425d-bdb5-e371d0aae2e2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_5e7279d3-a41f-425d-bdb5-e371d0aae2e2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_aa522766-0fa7-4cdb-9836-1c53191d35de" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:to="loc_us-gaap_NetIncomeLoss_aa522766-0fa7-4cdb-9836-1c53191d35de" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareAbstract_924f4e3d-17ed-4d51-ac74-78951336d5c9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EarningsPerShareAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_39898038-d090-402f-892c-41fe3e28ce79" xlink:to="loc_us-gaap_EarningsPerShareAbstract_924f4e3d-17ed-4d51-ac74-78951336d5c9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_35e9d8be-0cad-4b84-bdb0-619ab317658a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_924f4e3d-17ed-4d51-ac74-78951336d5c9" xlink:to="loc_us-gaap_EarningsPerShareBasic_35e9d8be-0cad-4b84-bdb0-619ab317658a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_f6e59c37-a5f5-4e31-b96f-c8b13502fb13" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_924f4e3d-17ed-4d51-ac74-78951336d5c9" xlink:to="loc_us-gaap_EarningsPerShareDiluted_f6e59c37-a5f5-4e31-b96f-c8b13502fb13" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_4aca8d9a-fecc-40ab-831e-5ff7cfd760d2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_924f4e3d-17ed-4d51-ac74-78951336d5c9" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_4aca8d9a-fecc-40ab-831e-5ff7cfd760d2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_c6c200d4-8724-46f5-9b34-4ee30a8bffdd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_924f4e3d-17ed-4d51-ac74-78951336d5c9" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_c6c200d4-8724-46f5-9b34-4ee30a8bffdd" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFEQUITY" xlink:type="simple" xlink:href="gral-20240630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFEQUITY"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_007a1416-da8f-48b2-b7eb-80966b63cf69" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_f448bca3-13ca-47eb-aaa6-1c802547d690" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_007a1416-da8f-48b2-b7eb-80966b63cf69" xlink:to="loc_us-gaap_StatementTable_f448bca3-13ca-47eb-aaa6-1c802547d690" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_e84df359-f15f-4f26-b5f6-8c9e13a4a26e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_f448bca3-13ca-47eb-aaa6-1c802547d690" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_e84df359-f15f-4f26-b5f6-8c9e13a4a26e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_e84df359-f15f-4f26-b5f6-8c9e13a4a26e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_e84df359-f15f-4f26-b5f6-8c9e13a4a26e" xlink:to="loc_us-gaap_EquityComponentDomain_e84df359-f15f-4f26-b5f6-8c9e13a4a26e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_37515da6-97e0-40dd-b2d7-5948b6707fe7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_e84df359-f15f-4f26-b5f6-8c9e13a4a26e" xlink:to="loc_us-gaap_EquityComponentDomain_37515da6-97e0-40dd-b2d7-5948b6707fe7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_bcfed742-0083-47c5-8eac-56e649cac533" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_37515da6-97e0-40dd-b2d7-5948b6707fe7" xlink:to="loc_us-gaap_CommonStockMember_bcfed742-0083-47c5-8eac-56e649cac533" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_MembersCapitalMember_74d2b401-ea30-439e-adb5-b52f8d916cc3" xlink:href="gral-20240630.xsd#gral_MembersCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_37515da6-97e0-40dd-b2d7-5948b6707fe7" xlink:to="loc_gral_MembersCapitalMember_74d2b401-ea30-439e-adb5-b52f8d916cc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_3da69b6e-03e7-4413-b613-7326eb4b15f2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_37515da6-97e0-40dd-b2d7-5948b6707fe7" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_3da69b6e-03e7-4413-b613-7326eb4b15f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_66e3a7af-a2dd-450e-87ff-fd79be334d7c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_37515da6-97e0-40dd-b2d7-5948b6707fe7" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_66e3a7af-a2dd-450e-87ff-fd79be334d7c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_70d05029-b0fd-4183-a3dc-8864364edfa5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_37515da6-97e0-40dd-b2d7-5948b6707fe7" xlink:to="loc_us-gaap_RetainedEarningsMember_70d05029-b0fd-4183-a3dc-8864364edfa5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_007a1416-da8f-48b2-b7eb-80966b63cf69" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_61fefd14-2647-4653-b3df-b2124fe06866" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_61fefd14-2647-4653-b3df-b2124fe06866" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_f8aa59b1-f8ae-4aae-95d5-5ae0d3f4de28" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_us-gaap_StockholdersEquity_f8aa59b1-f8ae-4aae-95d5-5ae0d3f4de28" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_b3790f73-6138-4530-a61b-8656d7357052" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_us-gaap_NetIncomeLoss_b3790f73-6138-4530-a61b-8656d7357052" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_020b2223-4c3b-4f95-b29b-8dffe3ab7c71" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_020b2223-4c3b-4f95-b29b-8dffe3ab7c71" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_96d82124-5b9c-46d8-99d7-8354bfca75f5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_96d82124-5b9c-46d8-99d7-8354bfca75f5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_StockholdersEquityDecreaseForTaxLiabilityDueToSpinOffTransaction_527ab3ea-3de0-491e-a287-3512af25d1d2" xlink:href="gral-20240630.xsd#gral_StockholdersEquityDecreaseForTaxLiabilityDueToSpinOffTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_gral_StockholdersEquityDecreaseForTaxLiabilityDueToSpinOffTransaction_527ab3ea-3de0-491e-a287-3512af25d1d2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_AdditionalPaidInCapitalReclassificationOfLiabilityClassifiedEquityAwardsFairValue_18c0b830-bb3b-440e-9ad9-6c38025bb80b" xlink:href="gral-20240630.xsd#gral_AdditionalPaidInCapitalReclassificationOfLiabilityClassifiedEquityAwardsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_gral_AdditionalPaidInCapitalReclassificationOfLiabilityClassifiedEquityAwardsFairValue_18c0b830-bb3b-440e-9ad9-6c38025bb80b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_StockholdersEquityNoteSpinoffTransactionFundingFromFormerParent_273209c0-a1ce-4ec9-85c8-dc045532279a" xlink:href="gral-20240630.xsd#gral_StockholdersEquityNoteSpinoffTransactionFundingFromFormerParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_gral_StockholdersEquityNoteSpinoffTransactionFundingFromFormerParent_273209c0-a1ce-4ec9-85c8-dc045532279a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_StockIssuedDuringPeriodSharesNewIssuesSpinoffTransaction_e9bff720-01e6-4364-853a-5cdb88f20efe" xlink:href="gral-20240630.xsd#gral_StockIssuedDuringPeriodSharesNewIssuesSpinoffTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_gral_StockIssuedDuringPeriodSharesNewIssuesSpinoffTransaction_e9bff720-01e6-4364-853a-5cdb88f20efe" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_StockIssuedDuringPeriodValueNewIssuesSpinoffTransaction_1e5331f7-eb01-4984-9374-d0d915468223" xlink:href="gral-20240630.xsd#gral_StockIssuedDuringPeriodValueNewIssuesSpinoffTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_11e2d716-d89c-44c3-8662-6c165347c1fd" xlink:to="loc_gral_StockIssuedDuringPeriodValueNewIssuesSpinoffTransaction_1e5331f7-eb01-4984-9374-d0d915468223" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_c8f07cab-311a-4f58-b943-2f451af8ef0f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_9d51a365-fd83-48e4-9140-3cd8616189b5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IncreaseDecreaseInMembersEquityRollForward_66ba3840-42e6-406f-9a0c-273531bb522f" xlink:href="gral-20240630.xsd#gral_IncreaseDecreaseInMembersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_007a1416-da8f-48b2-b7eb-80966b63cf69" xlink:to="loc_gral_IncreaseDecreaseInMembersEquityRollForward_66ba3840-42e6-406f-9a0c-273531bb522f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MembersEquity_1fbb9aa6-b653-4cae-8450-fbb38b431ffd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MembersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gral_IncreaseDecreaseInMembersEquityRollForward_66ba3840-42e6-406f-9a0c-273531bb522f" xlink:to="loc_us-gaap_MembersEquity_1fbb9aa6-b653-4cae-8450-fbb38b431ffd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_9b5376ac-1105-4a95-a627-1fe7362030d4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LimitedLiabilityCompanyLLCMembersEquityUnitBasedCompensation_e2736a2e-1c7b-4eb2-b9cb-7f2913f06e04" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LimitedLiabilityCompanyLLCMembersEquityUnitBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gral_IncreaseDecreaseInMembersEquityRollForward_66ba3840-42e6-406f-9a0c-273531bb522f" xlink:to="loc_us-gaap_LimitedLiabilityCompanyLLCMembersEquityUnitBasedCompensation_e2736a2e-1c7b-4eb2-b9cb-7f2913f06e04" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent_c6127cbe-f109-4f5a-947f-3b7090794b18" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_MembersEquityIncreaseFromMembersContribution_cc671ea6-3b2a-4612-8e3f-2d77489439df" xlink:href="gral-20240630.xsd#gral_MembersEquityIncreaseFromMembersContribution"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gral_IncreaseDecreaseInMembersEquityRollForward_66ba3840-42e6-406f-9a0c-273531bb522f" xlink:to="loc_gral_MembersEquityIncreaseFromMembersContribution_cc671ea6-3b2a-4612-8e3f-2d77489439df" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MembersEquity_5c6bdf31-3394-479c-9710-d885818a5c09" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MembersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOW" xlink:type="simple" xlink:href="gral-20240630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOW"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOW" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_6a4ce079-49a9-480f-811c-151b6cf4fe6c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_StatementTable_6a4ce079-49a9-480f-811c-151b6cf4fe6c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_a9ef4cff-91d6-46fd-beba-5bcece15eac4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_6a4ce079-49a9-480f-811c-151b6cf4fe6c" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_a9ef4cff-91d6-46fd-beba-5bcece15eac4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_a9ef4cff-91d6-46fd-beba-5bcece15eac4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_a9ef4cff-91d6-46fd-beba-5bcece15eac4" xlink:to="loc_us-gaap_RelatedPartyDomain_a9ef4cff-91d6-46fd-beba-5bcece15eac4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_c53271a0-1f0b-44f6-a44d-dcd1f30e3fcf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_a9ef4cff-91d6-46fd-beba-5bcece15eac4" xlink:to="loc_us-gaap_RelatedPartyDomain_c53271a0-1f0b-44f6-a44d-dcd1f30e3fcf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_4a2234ed-cff6-4b28-9ddb-4803caa9044c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_c53271a0-1f0b-44f6-a44d-dcd1f30e3fcf" xlink:to="loc_us-gaap_NonrelatedPartyMember_4a2234ed-cff6-4b28-9ddb-4803caa9044c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_8ce06a39-4706-4221-8a0d-c6708fcad6e9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_c53271a0-1f0b-44f6-a44d-dcd1f30e3fcf" xlink:to="loc_us-gaap_RelatedPartyMember_8ce06a39-4706-4221-8a0d-c6708fcad6e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_d3a50fef-a97b-497d-9d30-51fe89b37c68" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_d3a50fef-a97b-497d-9d30-51fe89b37c68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_287f8656-ab73-4fce-9629-7d4744e7d165" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_d3a50fef-a97b-497d-9d30-51fe89b37c68" xlink:to="loc_us-gaap_NetIncomeLoss_287f8656-ab73-4fce-9629-7d4744e7d165" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d3f44be1-5ade-4417-bc1e-6e0ae1b66db3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_d3a50fef-a97b-497d-9d30-51fe89b37c68" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d3f44be1-5ade-4417-bc1e-6e0ae1b66db3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_fa2a2b11-01a1-48f5-87e7-e64aff1ab06c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d3f44be1-5ade-4417-bc1e-6e0ae1b66db3" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_fa2a2b11-01a1-48f5-87e7-e64aff1ab06c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_4f4f10f0-ed4b-40f1-8a7d-e4d5ec3fc5a6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d3f44be1-5ade-4417-bc1e-6e0ae1b66db3" xlink:to="loc_us-gaap_Depreciation_4f4f10f0-ed4b-40f1-8a7d-e4d5ec3fc5a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_f9ea3cfa-747b-4cd1-b013-55ed050068c3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d3f44be1-5ade-4417-bc1e-6e0ae1b66db3" xlink:to="loc_us-gaap_ShareBasedCompensation_f9ea3cfa-747b-4cd1-b013-55ed050068c3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_CashFlowStatementCashPaidToSettleShareBasedPaymentAward_02ba904c-5f83-44cf-846a-5743feda1338" xlink:href="gral-20240630.xsd#gral_CashFlowStatementCashPaidToSettleShareBasedPaymentAward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d3f44be1-5ade-4417-bc1e-6e0ae1b66db3" xlink:to="loc_gral_CashFlowStatementCashPaidToSettleShareBasedPaymentAward_02ba904c-5f83-44cf-846a-5743feda1338" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_34859b28-7083-4f71-923e-7d1b800a2256" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d3f44be1-5ade-4417-bc1e-6e0ae1b66db3" xlink:to="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_34859b28-7083-4f71-923e-7d1b800a2256" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAndIntangibleAssetImpairment_f9d28dac-b64f-47eb-a727-7b623ea58f33" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillAndIntangibleAssetImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d3f44be1-5ade-4417-bc1e-6e0ae1b66db3" xlink:to="loc_us-gaap_GoodwillAndIntangibleAssetImpairment_f9d28dac-b64f-47eb-a727-7b623ea58f33" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncashIncomeExpense_5431f8e0-d93c-4c6d-96b1-0abcbb21622e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OtherNoncashIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d3f44be1-5ade-4417-bc1e-6e0ae1b66db3" xlink:to="loc_us-gaap_OtherNoncashIncomeExpense_5431f8e0-d93c-4c6d-96b1-0abcbb21622e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_995f0fda-23c6-4b85-981e-ec222b2653a8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_d3a50fef-a97b-497d-9d30-51fe89b37c68" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_995f0fda-23c6-4b85-981e-ec222b2653a8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_9025e278-cfdd-49bf-b3fd-5f5ab3277042" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_995f0fda-23c6-4b85-981e-ec222b2653a8" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_9025e278-cfdd-49bf-b3fd-5f5ab3277042" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidSupplies_5fe9a0af-f338-47fd-894a-8fadb14b6147" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInPrepaidSupplies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_995f0fda-23c6-4b85-981e-ec222b2653a8" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidSupplies_5fe9a0af-f338-47fd-894a-8fadb14b6147" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IncreaseDecreaseInOperatingLeaseRightOfUseAssetsAndLiabilitiesNet_1d5d7a2a-4f7c-4ecf-8c55-a9c075749550" xlink:href="gral-20240630.xsd#gral_IncreaseDecreaseInOperatingLeaseRightOfUseAssetsAndLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_995f0fda-23c6-4b85-981e-ec222b2653a8" xlink:to="loc_gral_IncreaseDecreaseInOperatingLeaseRightOfUseAssetsAndLiabilitiesNet_1d5d7a2a-4f7c-4ecf-8c55-a9c075749550" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidExpense_c4639c2d-5994-4192-af80-81b5b67cf351" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInPrepaidExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_995f0fda-23c6-4b85-981e-ec222b2653a8" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidExpense_c4639c2d-5994-4192-af80-81b5b67cf351" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayable_4cc1905e-fa47-4674-9bd4-6de525515747" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_995f0fda-23c6-4b85-981e-ec222b2653a8" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayable_4cc1905e-fa47-4674-9bd4-6de525515747" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_96658ea8-4d19-4462-8a16-b9049e85ca32" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_995f0fda-23c6-4b85-981e-ec222b2653a8" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_96658ea8-4d19-4462-8a16-b9049e85ca32" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_73a45e67-a2b0-4750-a60b-6817f726403a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_d3a50fef-a97b-497d-9d30-51fe89b37c68" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_73a45e67-a2b0-4750-a60b-6817f726403a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_7c136858-96d6-405c-9969-4802e8807724" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_7c136858-96d6-405c-9969-4802e8807724" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_17422a63-8e54-4f0c-88ac-cf158cb8cbfb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_7c136858-96d6-405c-9969-4802e8807724" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_17422a63-8e54-4f0c-88ac-cf158cb8cbfb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_fe366c6b-fb58-4a0a-9741-acf7774ea799" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_7c136858-96d6-405c-9969-4802e8807724" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_fe366c6b-fb58-4a0a-9741-acf7774ea799" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_4777eaf0-effd-4005-b8dd-8a80db6326fd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_4777eaf0-effd-4005-b8dd-8a80db6326fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromContributionsFromAffiliates_941654a4-d104-4e95-aeda-fa9e9f1cf36c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProceedsFromContributionsFromAffiliates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_4777eaf0-effd-4005-b8dd-8a80db6326fd" xlink:to="loc_us-gaap_ProceedsFromContributionsFromAffiliates_941654a4-d104-4e95-aeda-fa9e9f1cf36c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_2bbd87b8-c458-4941-b89d-248cb955529c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_4777eaf0-effd-4005-b8dd-8a80db6326fd" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_2bbd87b8-c458-4941-b89d-248cb955529c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_c9ade6a4-4507-49c8-a996-0a8d5051eca9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_c9ade6a4-4507-49c8-a996-0a8d5051eca9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_98f9e542-314f-42dd-9406-b8e702d82b16" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_98f9e542-314f-42dd-9406-b8e702d82b16" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_c43abe16-9f6c-4469-a633-aec00b90ccec" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_c43abe16-9f6c-4469-a633-aec00b90ccec" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_8d5522f8-99a5-4306-b0b7-f5e47b877aaf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_8d5522f8-99a5-4306-b0b7-f5e47b877aaf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_f30b9d63-4715-41c5-aaca-25bc482f9462" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAbstract_8733951e-953e-438b-af58-ad9dd566d6ac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAbstract_8733951e-953e-438b-af58-ad9dd566d6ac" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_351a45ad-596a-4dc3-887d-6562abb36f34" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAbstract_8733951e-953e-438b-af58-ad9dd566d6ac" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_351a45ad-596a-4dc3-887d-6562abb36f34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCash_f9868f0b-efe9-4009-8a92-0bb76574aac3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAbstract_8733951e-953e-438b-af58-ad9dd566d6ac" xlink:to="loc_us-gaap_RestrictedCash_f9868f0b-efe9-4009-8a92-0bb76574aac3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_c540b532-87b7-4d0d-9761-ebd7c0b4c1ef" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowElementsAbstract_09d0c2ec-b141-4ad8-b076-832712a431fa" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SupplementalCashFlowElementsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_47671421-cdbf-4321-b72d-673c7d6d35cb" xlink:to="loc_us-gaap_SupplementalCashFlowElementsAbstract_09d0c2ec-b141-4ad8-b076-832712a431fa" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_37503a83-7b04-42f0-a539-33c64b93126c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CapitalExpendituresIncurredButNotYetPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowElementsAbstract_09d0c2ec-b141-4ad8-b076-832712a431fa" xlink:to="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_37503a83-7b04-42f0-a539-33c64b93126c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasePayments_98fc5f93-2793-4591-a36c-40ce12ed3fc3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLeasePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowElementsAbstract_09d0c2ec-b141-4ad8-b076-832712a431fa" xlink:to="loc_us-gaap_OperatingLeasePayments_98fc5f93-2793-4591-a36c-40ce12ed3fc3" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/ORGANIZATIONANDDESCRIPTIONOFBUSINESSDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#ORGANIZATIONANDDESCRIPTIONOFBUSINESSDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/ORGANIZATIONANDDESCRIPTIONOFBUSINESSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable_df234b08-5496-4a86-9baa-cc6b5a4058db" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:to="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable_df234b08-5496-4a86-9baa-cc6b5a4058db" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_2897df89-f1bb-412b-92bd-c8d2344f9cf6" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable_df234b08-5496-4a86-9baa-cc6b5a4058db" xlink:to="loc_srt_OwnershipAxis_2897df89-f1bb-412b-92bd-c8d2344f9cf6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_2897df89-f1bb-412b-92bd-c8d2344f9cf6_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_2897df89-f1bb-412b-92bd-c8d2344f9cf6" xlink:to="loc_srt_OwnershipDomain_2897df89-f1bb-412b-92bd-c8d2344f9cf6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_f1f88666-a52c-435d-a230-00c78a97bc6d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_2897df89-f1bb-412b-92bd-c8d2344f9cf6" xlink:to="loc_srt_OwnershipDomain_f1f88666-a52c-435d-a230-00c78a97bc6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_SDGOpsLLCMember_92306681-b27a-4926-b850-d48266a17d6e" xlink:href="gral-20240630.xsd#gral_SDGOpsLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_f1f88666-a52c-435d-a230-00c78a97bc6d" xlink:to="loc_gral_SDGOpsLLCMember_92306681-b27a-4926-b850-d48266a17d6e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_GrailInc.Member_bd249469-a368-4684-b5c5-71c16056aff5" xlink:href="gral-20240630.xsd#gral_GrailInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_f1f88666-a52c-435d-a230-00c78a97bc6d" xlink:to="loc_gral_GrailInc.Member_bd249469-a368-4684-b5c5-71c16056aff5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_cf20ca08-d96f-482c-bf84-da7554bc4904" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable_df234b08-5496-4a86-9baa-cc6b5a4058db" xlink:to="loc_srt_CounterpartyNameAxis_cf20ca08-d96f-482c-bf84-da7554bc4904" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_cf20ca08-d96f-482c-bf84-da7554bc4904_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_cf20ca08-d96f-482c-bf84-da7554bc4904" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_cf20ca08-d96f-482c-bf84-da7554bc4904_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_a6a58394-a9ce-458c-85eb-876d4af1abb5" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_cf20ca08-d96f-482c-bf84-da7554bc4904" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_a6a58394-a9ce-458c-85eb-876d4af1abb5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_eaaf0c13-62f6-4e8c-a8c3-33fdcc233fd3" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_a6a58394-a9ce-458c-85eb-876d4af1abb5" xlink:to="loc_gral_IlluminaInc.Member_eaaf0c13-62f6-4e8c-a8c3-33fdcc233fd3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_dfb9d281-2618-46bc-8f56-468aa1bf210c" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable_df234b08-5496-4a86-9baa-cc6b5a4058db" xlink:to="loc_dei_LegalEntityAxis_dfb9d281-2618-46bc-8f56-468aa1bf210c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_dfb9d281-2618-46bc-8f56-468aa1bf210c_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_dfb9d281-2618-46bc-8f56-468aa1bf210c" xlink:to="loc_dei_EntityDomain_dfb9d281-2618-46bc-8f56-468aa1bf210c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_4fef2af5-c29a-4339-b0e6-67a449cc974b" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_dfb9d281-2618-46bc-8f56-468aa1bf210c" xlink:to="loc_dei_EntityDomain_4fef2af5-c29a-4339-b0e6-67a449cc974b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_0e6f1c1c-f54c-40b0-b448-44fde6120884" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_4fef2af5-c29a-4339-b0e6-67a449cc974b" xlink:to="loc_gral_IlluminaInc.Member_0e6f1c1c-f54c-40b0-b448-44fde6120884" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPManagingMemberOrGeneralPartnerOwnershipInterest_c05708d5-65c7-4e5d-88af-ec1d43a6646e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPManagingMemberOrGeneralPartnerOwnershipInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:to="loc_us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPManagingMemberOrGeneralPartnerOwnershipInterest_c05708d5-65c7-4e5d-88af-ec1d43a6646e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_StockholdersEquityNoteSpinoffTransactionPercentOfSharesOutstandingDistributed_d261dee7-8ba0-4e51-b016-5d506fa5f66d" xlink:href="gral-20240630.xsd#gral_StockholdersEquityNoteSpinoffTransactionPercentOfSharesOutstandingDistributed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:to="loc_gral_StockholdersEquityNoteSpinoffTransactionPercentOfSharesOutstandingDistributed_d261dee7-8ba0-4e51-b016-5d506fa5f66d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_70447ea2-9c1c-49e5-a2f9-745fc4ebab8e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:to="loc_us-gaap_CommonStockSharesIssued_70447ea2-9c1c-49e5-a2f9-745fc4ebab8e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityOwnershipPercentageExcludingConsolidatedEntityAndEquityMethodInvestee_fae31808-a785-4762-9381-c135e541c755" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityOwnershipPercentageExcludingConsolidatedEntityAndEquityMethodInvestee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:to="loc_us-gaap_EquityOwnershipPercentageExcludingConsolidatedEntityAndEquityMethodInvestee_fae31808-a785-4762-9381-c135e541c755" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromContributionsFromAffiliates_dc2a6636-6d1e-471c-bddd-a72532c2a4dd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProceedsFromContributionsFromAffiliates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:to="loc_us-gaap_ProceedsFromContributionsFromAffiliates_dc2a6636-6d1e-471c-bddd-a72532c2a4dd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ContributionFromAffiliateClawbackTerm_05b3b096-28b6-4f71-b738-185fe3948d25" xlink:href="gral-20240630.xsd#gral_ContributionFromAffiliateClawbackTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:to="loc_gral_ContributionFromAffiliateClawbackTerm_05b3b096-28b6-4f71-b738-185fe3948d25" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_34cb895f-781e-4350-b80f-30968d6ef23b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_34cb895f-781e-4350-b80f-30968d6ef23b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_StockholdersEquityNoteSpinoffTransactionNumberOfSharesDistributed_ac300828-96e4-4c4d-9def-fb9c7d3500eb" xlink:href="gral-20240630.xsd#gral_StockholdersEquityNoteSpinoffTransactionNumberOfSharesDistributed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems_c6cc3e47-e82f-416a-92e2-416f0713dc70" xlink:to="loc_gral_StockholdersEquityNoteSpinoffTransactionNumberOfSharesDistributed_ac300828-96e4-4c4d-9def-fb9c7d3500eb" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_a2ef6c4e-df9e-4aa0-b897-59a8ec22bfe4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_2cfc4713-60cd-4e8e-9d13-d50123aea08a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_a2ef6c4e-df9e-4aa0-b897-59a8ec22bfe4" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_2cfc4713-60cd-4e8e-9d13-d50123aea08a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_c60d6c62-407d-4076-a8e2-98613cb29cd4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_2cfc4713-60cd-4e8e-9d13-d50123aea08a" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_c60d6c62-407d-4076-a8e2-98613cb29cd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_c60d6c62-407d-4076-a8e2-98613cb29cd4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_c60d6c62-407d-4076-a8e2-98613cb29cd4" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_c60d6c62-407d-4076-a8e2-98613cb29cd4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_3315f1bc-5a72-45fa-9adb-70ca04321816" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_c60d6c62-407d-4076-a8e2-98613cb29cd4" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_3315f1bc-5a72-45fa-9adb-70ca04321816" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_aba789b5-20d2-4e52-9f26-edeba88141a4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_3315f1bc-5a72-45fa-9adb-70ca04321816" xlink:to="loc_us-gaap_MoneyMarketFundsMember_aba789b5-20d2-4e52-9f26-edeba88141a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_976557f0-aab5-40f0-b9e9-6824bc05b79e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_2cfc4713-60cd-4e8e-9d13-d50123aea08a" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_976557f0-aab5-40f0-b9e9-6824bc05b79e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_976557f0-aab5-40f0-b9e9-6824bc05b79e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_976557f0-aab5-40f0-b9e9-6824bc05b79e" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_976557f0-aab5-40f0-b9e9-6824bc05b79e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_e1a8755a-f707-434d-90f5-effd01c75817" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_976557f0-aab5-40f0-b9e9-6824bc05b79e" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_e1a8755a-f707-434d-90f5-effd01c75817" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_2018ddd7-bd80-4c55-9c9b-c37ad1ad242e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_e1a8755a-f707-434d-90f5-effd01c75817" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_2018ddd7-bd80-4c55-9c9b-c37ad1ad242e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ddc9c0aa-78e3-4453-a0d6-e0deaf4bd1eb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_2cfc4713-60cd-4e8e-9d13-d50123aea08a" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ddc9c0aa-78e3-4453-a0d6-e0deaf4bd1eb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ddc9c0aa-78e3-4453-a0d6-e0deaf4bd1eb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ddc9c0aa-78e3-4453-a0d6-e0deaf4bd1eb" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ddc9c0aa-78e3-4453-a0d6-e0deaf4bd1eb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_e5ee6b59-2d9a-46d6-aefb-cab54b1f9537" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ddc9c0aa-78e3-4453-a0d6-e0deaf4bd1eb" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_e5ee6b59-2d9a-46d6-aefb-cab54b1f9537" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_09bc0be6-691d-4916-8eb0-6ec42c3e2184" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_e5ee6b59-2d9a-46d6-aefb-cab54b1f9537" xlink:to="loc_us-gaap_SalesRevenueNetMember_09bc0be6-691d-4916-8eb0-6ec42c3e2184" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_d30ef3e3-9813-4537-b0e9-9250154667ac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_e5ee6b59-2d9a-46d6-aefb-cab54b1f9537" xlink:to="loc_us-gaap_AccountsReceivableMember_d30ef3e3-9813-4537-b0e9-9250154667ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_50143006-8c17-49b1-b4ba-8b4d8c8d485b" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_2cfc4713-60cd-4e8e-9d13-d50123aea08a" xlink:to="loc_srt_MajorCustomersAxis_50143006-8c17-49b1-b4ba-8b4d8c8d485b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_50143006-8c17-49b1-b4ba-8b4d8c8d485b_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_50143006-8c17-49b1-b4ba-8b4d8c8d485b" xlink:to="loc_srt_NameOfMajorCustomerDomain_50143006-8c17-49b1-b4ba-8b4d8c8d485b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_6ca12443-4633-4132-8897-abba27cd5cbb" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_50143006-8c17-49b1-b4ba-8b4d8c8d485b" xlink:to="loc_srt_NameOfMajorCustomerDomain_6ca12443-4633-4132-8897-abba27cd5cbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_OneCustomerMember_24d1385c-2430-4fb1-94da-a6115aabd6f9" xlink:href="gral-20240630.xsd#gral_OneCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_6ca12443-4633-4132-8897-abba27cd5cbb" xlink:to="loc_gral_OneCustomerMember_24d1385c-2430-4fb1-94da-a6115aabd6f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_da1744b3-7c01-4bac-a825-3915a7d3c2b8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_2cfc4713-60cd-4e8e-9d13-d50123aea08a" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_da1744b3-7c01-4bac-a825-3915a7d3c2b8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_da1744b3-7c01-4bac-a825-3915a7d3c2b8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_da1744b3-7c01-4bac-a825-3915a7d3c2b8" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_da1744b3-7c01-4bac-a825-3915a7d3c2b8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_8c454a4b-25b6-4cda-acfa-89ec4aff19b0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_da1744b3-7c01-4bac-a825-3915a7d3c2b8" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_8c454a4b-25b6-4cda-acfa-89ec4aff19b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_f08c5fc1-7214-46bb-8856-5e31c9dfade7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DomesticCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_8c454a4b-25b6-4cda-acfa-89ec4aff19b0" xlink:to="loc_us-gaap_DomesticCountryMember_f08c5fc1-7214-46bb-8856-5e31c9dfade7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_652b35a9-e040-4aef-95f0-6467bfcfe115" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_8c454a4b-25b6-4cda-acfa-89ec4aff19b0" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_652b35a9-e040-4aef-95f0-6467bfcfe115" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_54983917-bf30-4357-94a2-82ff3a09112a" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_2cfc4713-60cd-4e8e-9d13-d50123aea08a" xlink:to="loc_dei_LegalEntityAxis_54983917-bf30-4357-94a2-82ff3a09112a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_54983917-bf30-4357-94a2-82ff3a09112a_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_54983917-bf30-4357-94a2-82ff3a09112a" xlink:to="loc_dei_EntityDomain_54983917-bf30-4357-94a2-82ff3a09112a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_df5af62f-c540-4561-95e8-f5844def8f99" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_54983917-bf30-4357-94a2-82ff3a09112a" xlink:to="loc_dei_EntityDomain_df5af62f-c540-4561-95e8-f5844def8f99" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_5ed87058-9ed1-4175-95d9-ee27539638e0" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_df5af62f-c540-4561-95e8-f5844def8f99" xlink:to="loc_gral_IlluminaInc.Member_5ed87058-9ed1-4175-95d9-ee27539638e0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_3d4399e8-3166-4a14-a612-0af148a1acf5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_a2ef6c4e-df9e-4aa0-b897-59a8ec22bfe4" xlink:to="loc_us-gaap_OperatingLossCarryforwards_3d4399e8-3166-4a14-a612-0af148a1acf5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_bdffb71f-999f-4df5-996f-ee6b4bb92f81" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_a2ef6c4e-df9e-4aa0-b897-59a8ec22bfe4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_bdffb71f-999f-4df5-996f-ee6b4bb92f81" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedResearchAndDevelopmentCosts_155fee0a-eeaf-4cfd-b898-99c54f4fde9a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedResearchAndDevelopmentCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_a2ef6c4e-df9e-4aa0-b897-59a8ec22bfe4" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedResearchAndDevelopmentCosts_155fee0a-eeaf-4cfd-b898-99c54f4fde9a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsAtCarryingValue_fb1711d3-22a9-4a2e-ace1-d9b071d19c16" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_a2ef6c4e-df9e-4aa0-b897-59a8ec22bfe4" xlink:to="loc_us-gaap_CashEquivalentsAtCarryingValue_fb1711d3-22a9-4a2e-ace1-d9b071d19c16" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromContributionsFromAffiliates_1278459c-dd67-4105-b9f8-8f95c95ed5b0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProceedsFromContributionsFromAffiliates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_a2ef6c4e-df9e-4aa0-b897-59a8ec22bfe4" xlink:to="loc_us-gaap_ProceedsFromContributionsFromAffiliates_1278459c-dd67-4105-b9f8-8f95c95ed5b0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_b391421a-1b0e-4c80-8e53-d17ae1de1685" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_a2ef6c4e-df9e-4aa0-b897-59a8ec22bfe4" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_b391421a-1b0e-4c80-8e53-d17ae1de1685" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/GOODWILLANDINTANGIBLEASSETSGoodwillnarrativeDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#GOODWILLANDINTANGIBLEASSETSGoodwillnarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/GOODWILLANDINTANGIBLEASSETSGoodwillnarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_457692ff-5d2e-4027-9466-69e14ae7dae4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_3afac6f4-09fa-4042-8bb0-dcc89f0d50c4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_457692ff-5d2e-4027-9466-69e14ae7dae4" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_3afac6f4-09fa-4042-8bb0-dcc89f0d50c4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_e7dcdc48-fc23-4515-ad18-915028a6c38e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_3afac6f4-09fa-4042-8bb0-dcc89f0d50c4" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_e7dcdc48-fc23-4515-ad18-915028a6c38e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_e7dcdc48-fc23-4515-ad18-915028a6c38e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_e7dcdc48-fc23-4515-ad18-915028a6c38e" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_e7dcdc48-fc23-4515-ad18-915028a6c38e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_d143c2ff-a535-4050-a687-0d3bd2377b86" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_e7dcdc48-fc23-4515-ad18-915028a6c38e" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_d143c2ff-a535-4050-a687-0d3bd2377b86" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputDiscountRateMember_fbaf9ccd-a9e0-46fc-a856-7e2f3724da6c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputDiscountRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_d143c2ff-a535-4050-a687-0d3bd2377b86" xlink:to="loc_us-gaap_MeasurementInputDiscountRateMember_fbaf9ccd-a9e0-46fc-a856-7e2f3724da6c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_891cb408-c3ea-4f3e-ad46-2b90395eaa7a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_3afac6f4-09fa-4042-8bb0-dcc89f0d50c4" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_891cb408-c3ea-4f3e-ad46-2b90395eaa7a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_891cb408-c3ea-4f3e-ad46-2b90395eaa7a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_891cb408-c3ea-4f3e-ad46-2b90395eaa7a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_891cb408-c3ea-4f3e-ad46-2b90395eaa7a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_721799de-3081-4a41-9009-e37ff2e8ea97" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_891cb408-c3ea-4f3e-ad46-2b90395eaa7a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_721799de-3081-4a41-9009-e37ff2e8ea97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_78258e25-943c-4102-934e-40df8771d913" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_721799de-3081-4a41-9009-e37ff2e8ea97" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_78258e25-943c-4102-934e-40df8771d913" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_GoodwillMeasurementInput_f5f8717b-10a6-4299-9403-60afd0fcab12" xlink:href="gral-20240630.xsd#gral_GoodwillMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_457692ff-5d2e-4027-9466-69e14ae7dae4" xlink:to="loc_gral_GoodwillMeasurementInput_f5f8717b-10a6-4299-9403-60afd0fcab12" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_5bbbfb44-6622-4741-9ed2-24259999cf63" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GoodwillImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_457692ff-5d2e-4027-9466-69e14ae7dae4" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_5bbbfb44-6622-4741-9ed2-24259999cf63" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofintangibleassetsDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#GOODWILLANDINTANGIBLEASSETSScheduleofintangibleassetsDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/GOODWILLANDINTANGIBLEASSETSScheduleofintangibleassetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_a1b919d2-dea6-4be4-ab5b-3ec720c2cfe9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_c3b09665-422f-4c22-8ed3-c63f196dae78" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_a1b919d2-dea6-4be4-ab5b-3ec720c2cfe9" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_c3b09665-422f-4c22-8ed3-c63f196dae78" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1965b285-b2c4-4391-adcb-bfcf76d3ebf4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_c3b09665-422f-4c22-8ed3-c63f196dae78" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1965b285-b2c4-4391-adcb-bfcf76d3ebf4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1965b285-b2c4-4391-adcb-bfcf76d3ebf4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1965b285-b2c4-4391-adcb-bfcf76d3ebf4" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1965b285-b2c4-4391-adcb-bfcf76d3ebf4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_de60a27b-d890-4269-8163-80c445604954" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1965b285-b2c4-4391-adcb-bfcf76d3ebf4" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_de60a27b-d890-4269-8163-80c445604954" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_f1a6f194-c881-4f9a-be86-df7aa3529745" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_de60a27b-d890-4269-8163-80c445604954" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_f1a6f194-c881-4f9a-be86-df7aa3529745" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_d2adc244-f4c5-45e0-a0e4-6924b7a4e9e6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_de60a27b-d890-4269-8163-80c445604954" xlink:to="loc_us-gaap_TradeNamesMember_d2adc244-f4c5-45e0-a0e4-6924b7a4e9e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_555b4f9d-8f6d-45fb-82c2-e372f059d1f8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_a1b919d2-dea6-4be4-ab5b-3ec720c2cfe9" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_555b4f9d-8f6d-45fb-82c2-e372f059d1f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_b202744f-f071-4d8b-8475-5dc71431f05e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_555b4f9d-8f6d-45fb-82c2-e372f059d1f8" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_b202744f-f071-4d8b-8475-5dc71431f05e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_8527fbf7-7119-4910-9912-e36a0ec6666b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_555b4f9d-8f6d-45fb-82c2-e372f059d1f8" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_8527fbf7-7119-4910-9912-e36a0ec6666b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_FiniteLivedIntangibleAssetsAccumulatedImpairment_66dec75f-66c3-4a65-b342-f5cb88877e75" xlink:href="gral-20240630.xsd#gral_FiniteLivedIntangibleAssetsAccumulatedImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_555b4f9d-8f6d-45fb-82c2-e372f059d1f8" xlink:to="loc_gral_FiniteLivedIntangibleAssetsAccumulatedImpairment_66dec75f-66c3-4a65-b342-f5cb88877e75" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_a55fdc8c-b18d-4d3c-a2d2-cf6278b15bcf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_555b4f9d-8f6d-45fb-82c2-e372f059d1f8" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_a55fdc8c-b18d-4d3c-a2d2-cf6278b15bcf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_1377c98e-50a4-4393-ae32-97e1a2e14f62" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_54d5cc0c-af87-4c50-b340-95a90f0420e5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_1377c98e-50a4-4393-ae32-97e1a2e14f62" xlink:to="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_54d5cc0c-af87-4c50-b340-95a90f0420e5" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_1dfdc881-cbb4-4e96-bac1-1ec63d08f6b3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_54d5cc0c-af87-4c50-b340-95a90f0420e5" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_1dfdc881-cbb4-4e96-bac1-1ec63d08f6b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_1dfdc881-cbb4-4e96-bac1-1ec63d08f6b3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_1dfdc881-cbb4-4e96-bac1-1ec63d08f6b3" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_1dfdc881-cbb4-4e96-bac1-1ec63d08f6b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_a67145fe-ec29-4d61-9c57-8b77bd649931" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_1dfdc881-cbb4-4e96-bac1-1ec63d08f6b3" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_a67145fe-ec29-4d61-9c57-8b77bd649931" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_9a142f76-8eca-4d25-87e3-bc59d5161b82" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_a67145fe-ec29-4d61-9c57-8b77bd649931" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_9a142f76-8eca-4d25-87e3-bc59d5161b82" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IndefiniteLivedIntangibleAssetsGrossExcludingGoodwill_1af447f3-2510-4d86-a3f9-308b2bd290b6" xlink:href="gral-20240630.xsd#gral_IndefiniteLivedIntangibleAssetsGrossExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_1377c98e-50a4-4393-ae32-97e1a2e14f62" xlink:to="loc_gral_IndefiniteLivedIntangibleAssetsGrossExcludingGoodwill_1af447f3-2510-4d86-a3f9-308b2bd290b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IndefiniteLivedIntangibleAssetsAccumulatedImpairment_3c598c95-c5f0-4557-8044-50eb90a1bae3" xlink:href="gral-20240630.xsd#gral_IndefiniteLivedIntangibleAssetsAccumulatedImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_1377c98e-50a4-4393-ae32-97e1a2e14f62" xlink:to="loc_gral_IndefiniteLivedIntangibleAssetsAccumulatedImpairment_3c598c95-c5f0-4557-8044-50eb90a1bae3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_48d9db45-886e-4011-b209-ed7def7a678f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_1377c98e-50a4-4393-ae32-97e1a2e14f62" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_48d9db45-886e-4011-b209-ed7def7a678f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/GOODWILLANDINTANGIBLEASSETSIntangibleassetsnarrativeDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#GOODWILLANDINTANGIBLEASSETSIntangibleassetsnarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/GOODWILLANDINTANGIBLEASSETSIntangibleassetsnarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_3c18f9b5-62d5-4926-9d06-e5f41fe09e57" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_91c1d644-d773-4206-9c84-0cdd9491441e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_3c18f9b5-62d5-4926-9d06-e5f41fe09e57" xlink:to="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_91c1d644-d773-4206-9c84-0cdd9491441e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_b72a73fa-4c3f-4a62-9248-cb81f4c5def5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_91c1d644-d773-4206-9c84-0cdd9491441e" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_b72a73fa-4c3f-4a62-9248-cb81f4c5def5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_b72a73fa-4c3f-4a62-9248-cb81f4c5def5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_b72a73fa-4c3f-4a62-9248-cb81f4c5def5" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_b72a73fa-4c3f-4a62-9248-cb81f4c5def5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_dfbe870c-843a-4aa8-8c3f-7f8d809f968c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_b72a73fa-4c3f-4a62-9248-cb81f4c5def5" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_dfbe870c-843a-4aa8-8c3f-7f8d809f968c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_c38d091f-8130-44f7-9c07-03b12fad8824" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_dfbe870c-843a-4aa8-8c3f-7f8d809f968c" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_c38d091f-8130-44f7-9c07-03b12fad8824" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_c47e1286-ba0d-4578-a809-95928a00fd2d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_91c1d644-d773-4206-9c84-0cdd9491441e" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_c47e1286-ba0d-4578-a809-95928a00fd2d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_c47e1286-ba0d-4578-a809-95928a00fd2d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_c47e1286-ba0d-4578-a809-95928a00fd2d" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_c47e1286-ba0d-4578-a809-95928a00fd2d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_15c614b9-3d1c-41e8-90dc-2d3755061d4e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_c47e1286-ba0d-4578-a809-95928a00fd2d" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_15c614b9-3d1c-41e8-90dc-2d3755061d4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputDiscountRateMember_63405eed-4f41-441b-b887-761c8b254db5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_MeasurementInputDiscountRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_15c614b9-3d1c-41e8-90dc-2d3755061d4e" xlink:to="loc_us-gaap_MeasurementInputDiscountRateMember_63405eed-4f41-441b-b887-761c8b254db5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_58298068-6ca2-4751-bee5-7677c758b955" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfIndefiniteLivedIntangibleAssetsTable_91c1d644-d773-4206-9c84-0cdd9491441e" xlink:to="loc_dei_LegalEntityAxis_58298068-6ca2-4751-bee5-7677c758b955" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_58298068-6ca2-4751-bee5-7677c758b955_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_58298068-6ca2-4751-bee5-7677c758b955" xlink:to="loc_dei_EntityDomain_58298068-6ca2-4751-bee5-7677c758b955_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_d402a3cd-c160-4033-9a18-b9e7c522559e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_58298068-6ca2-4751-bee5-7677c758b955" xlink:to="loc_dei_EntityDomain_d402a3cd-c160-4033-9a18-b9e7c522559e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_f0f7569a-b469-4098-8b5a-ddd24be539fb" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_d402a3cd-c160-4033-9a18-b9e7c522559e" xlink:to="loc_gral_IlluminaInc.Member_f0f7569a-b469-4098-8b5a-ddd24be539fb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IntangibleAssetMeasurementInput_2bc4bdf9-3f1a-418a-abb7-2568d5bc956e" xlink:href="gral-20240630.xsd#gral_IntangibleAssetMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_3c18f9b5-62d5-4926-9d06-e5f41fe09e57" xlink:to="loc_gral_IntangibleAssetMeasurementInput_2bc4bdf9-3f1a-418a-abb7-2568d5bc956e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill_81fcc9f9-e42a-49c1-8e61-cf09cb052b2c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_3c18f9b5-62d5-4926-9d06-e5f41fe09e57" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill_81fcc9f9-e42a-49c1-8e61-cf09cb052b2c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_7b1849e0-ec54-46a4-a0f0-9f8d56a46b22" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassLineItems_3c18f9b5-62d5-4926-9d06-e5f41fe09e57" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_7b1849e0-ec54-46a4-a0f0-9f8d56a46b22" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/BALANCESHEETCOMPONENTSScheduleofaccountsreceivablenetDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#BALANCESHEETCOMPONENTSScheduleofaccountsreceivablenetDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/BALANCESHEETCOMPONENTSScheduleofaccountsreceivablenetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_3f18fad1-ba73-452c-8d29-094ba1d1b5c4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsNotesAndLoansReceivableLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_19682568-092e-4dec-b106-ae6a51b81ca5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_3f18fad1-ba73-452c-8d29-094ba1d1b5c4" xlink:to="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_19682568-092e-4dec-b106-ae6a51b81ca5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_44f82666-ef70-439d-a26f-25ab7dcc935c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_19682568-092e-4dec-b106-ae6a51b81ca5" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_44f82666-ef70-439d-a26f-25ab7dcc935c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_44f82666-ef70-439d-a26f-25ab7dcc935c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_44f82666-ef70-439d-a26f-25ab7dcc935c" xlink:to="loc_us-gaap_RelatedPartyDomain_44f82666-ef70-439d-a26f-25ab7dcc935c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_382e6833-a074-47db-94b1-50d77c310817" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_44f82666-ef70-439d-a26f-25ab7dcc935c" xlink:to="loc_us-gaap_RelatedPartyDomain_382e6833-a074-47db-94b1-50d77c310817" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_090beb31-ff5f-4fd2-94f7-a7413624ac68" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_382e6833-a074-47db-94b1-50d77c310817" xlink:to="loc_us-gaap_NonrelatedPartyMember_090beb31-ff5f-4fd2-94f7-a7413624ac68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableGrossCurrent_8da5ce1a-f96c-4514-98f2-ff4e4106df84" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsReceivableGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_3f18fad1-ba73-452c-8d29-094ba1d1b5c4" xlink:to="loc_us-gaap_AccountsReceivableGrossCurrent_8da5ce1a-f96c-4514-98f2-ff4e4106df84" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_c849a5cb-a038-490b-869a-358e696b56c0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_3f18fad1-ba73-452c-8d29-094ba1d1b5c4" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_c849a5cb-a038-490b-869a-358e696b56c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_09a3a0de-f787-4158-ad24-5071d375f4f4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_3f18fad1-ba73-452c-8d29-094ba1d1b5c4" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_09a3a0de-f787-4158-ad24-5071d375f4f4" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/STOCKBASEDCOMPENSATIONStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#STOCKBASEDCOMPENSATIONStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/STOCKBASEDCOMPENSATIONStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_81d9cf3a-8778-4cff-8b23-0f3ac1c24cbe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_339ed079-4c27-4c54-8aa8-7cf5ef75f18b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_81d9cf3a-8778-4cff-8b23-0f3ac1c24cbe" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_339ed079-4c27-4c54-8aa8-7cf5ef75f18b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_26a0d869-b699-4edb-bd3a-7d8afe86096d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_339ed079-4c27-4c54-8aa8-7cf5ef75f18b" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_26a0d869-b699-4edb-bd3a-7d8afe86096d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_26a0d869-b699-4edb-bd3a-7d8afe86096d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_26a0d869-b699-4edb-bd3a-7d8afe86096d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_26a0d869-b699-4edb-bd3a-7d8afe86096d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_6430a191-510c-41c3-9e36-eb12c25cbbb7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_26a0d869-b699-4edb-bd3a-7d8afe86096d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_6430a191-510c-41c3-9e36-eb12c25cbbb7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_CostOfScreeningRevenueMember_c2bef95b-e179-410e-97e8-89f3193b12a6" xlink:href="gral-20240630.xsd#gral_CostOfScreeningRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_6430a191-510c-41c3-9e36-eb12c25cbbb7" xlink:to="loc_gral_CostOfScreeningRevenueMember_c2bef95b-e179-410e-97e8-89f3193b12a6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_CostOfDevelopmentServicesRevenueMember_a2fb5ed9-54ab-48b2-9222-66d9ba2e500b" xlink:href="gral-20240630.xsd#gral_CostOfDevelopmentServicesRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_6430a191-510c-41c3-9e36-eb12c25cbbb7" xlink:to="loc_gral_CostOfDevelopmentServicesRevenueMember_a2fb5ed9-54ab-48b2-9222-66d9ba2e500b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_0a296b99-5e87-42d3-a877-3cf2294337f2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_6430a191-510c-41c3-9e36-eb12c25cbbb7" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_0a296b99-5e87-42d3-a877-3cf2294337f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_5fcfe456-6620-4568-89fd-65059e125f4a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SellingAndMarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_6430a191-510c-41c3-9e36-eb12c25cbbb7" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_5fcfe456-6620-4568-89fd-65059e125f4a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_13df7e37-7754-4521-af34-b46e92f50251" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_6430a191-510c-41c3-9e36-eb12c25cbbb7" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_13df7e37-7754-4521-af34-b46e92f50251" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_e3fa1e19-d3c3-4560-8b6b-803dde820e5a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_81d9cf3a-8778-4cff-8b23-0f3ac1c24cbe" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_e3fa1e19-d3c3-4560-8b6b-803dde820e5a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_9ae96477-6d17-431c-9842-283c8e0c445a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_81d9cf3a-8778-4cff-8b23-0f3ac1c24cbe" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_9ae96477-6d17-431c-9842-283c8e0c445a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_3d129b32-1ffb-4082-ade1-c88a223a6618" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_81d9cf3a-8778-4cff-8b23-0f3ac1c24cbe" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_3d129b32-1ffb-4082-ade1-c88a223a6618" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/STOCKBASEDCOMPENSATION2024IncentiveAwardPlanDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#STOCKBASEDCOMPENSATION2024IncentiveAwardPlanDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/STOCKBASEDCOMPENSATION2024IncentiveAwardPlanDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2d2ffaa3-971e-417e-bbcf-0bdf0c93a603" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4315fc7b-bdaa-4efb-aa37-6e7235cd57bc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2d2ffaa3-971e-417e-bbcf-0bdf0c93a603" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4315fc7b-bdaa-4efb-aa37-6e7235cd57bc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_25a19211-55d3-4004-90d7-00e4a0db9c72" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4315fc7b-bdaa-4efb-aa37-6e7235cd57bc" xlink:to="loc_us-gaap_PlanNameAxis_25a19211-55d3-4004-90d7-00e4a0db9c72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_25a19211-55d3-4004-90d7-00e4a0db9c72_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_25a19211-55d3-4004-90d7-00e4a0db9c72" xlink:to="loc_us-gaap_PlanNameDomain_25a19211-55d3-4004-90d7-00e4a0db9c72_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_9c8437ff-0d4c-4d8d-971d-cdadc079e608" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_25a19211-55d3-4004-90d7-00e4a0db9c72" xlink:to="loc_us-gaap_PlanNameDomain_9c8437ff-0d4c-4d8d-971d-cdadc079e608" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_TheGRAILInc.2024IncentiveAwardPlanMember_83d47ca9-c855-4ecf-ad13-a0a9e999175a" xlink:href="gral-20240630.xsd#gral_TheGRAILInc.2024IncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_9c8437ff-0d4c-4d8d-971d-cdadc079e608" xlink:to="loc_gral_TheGRAILInc.2024IncentiveAwardPlanMember_83d47ca9-c855-4ecf-ad13-a0a9e999175a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_6424b8dc-8625-4291-a558-a4fcce04b452" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2d2ffaa3-971e-417e-bbcf-0bdf0c93a603" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_6424b8dc-8625-4291-a558-a4fcce04b452" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationByShareBasedPaymentAwardPercentageOfOutstandingStockAnnualIncrease_e1cfdbff-239e-4892-84ab-7d55b54d693d" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationByShareBasedPaymentAwardPercentageOfOutstandingStockAnnualIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_2d2ffaa3-971e-417e-bbcf-0bdf0c93a603" xlink:to="loc_gral_ShareBasedCompensationByShareBasedPaymentAwardPercentageOfOutstandingStockAnnualIncrease_e1cfdbff-239e-4892-84ab-7d55b54d693d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/STOCKBASEDCOMPENSATIONRSUActivityDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#STOCKBASEDCOMPENSATIONRSUActivityDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/STOCKBASEDCOMPENSATIONRSUActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9b5dfa4a-77c2-4abb-b4bb-3b808e06bd4f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ac499624-c4ec-4eec-83e9-6b17e5067d1f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9b5dfa4a-77c2-4abb-b4bb-3b808e06bd4f" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ac499624-c4ec-4eec-83e9-6b17e5067d1f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_91744070-1acd-42c1-aa81-36cf9b0a9299" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ac499624-c4ec-4eec-83e9-6b17e5067d1f" xlink:to="loc_us-gaap_AwardTypeAxis_91744070-1acd-42c1-aa81-36cf9b0a9299" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_91744070-1acd-42c1-aa81-36cf9b0a9299_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_91744070-1acd-42c1-aa81-36cf9b0a9299" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_91744070-1acd-42c1-aa81-36cf9b0a9299_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_74779f58-57e6-4497-8663-4dbd81398aa3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_91744070-1acd-42c1-aa81-36cf9b0a9299" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_74779f58-57e6-4497-8663-4dbd81398aa3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_c4375ff9-fe9a-4c58-a4cd-70cfe21b29e2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_74779f58-57e6-4497-8663-4dbd81398aa3" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_c4375ff9-fe9a-4c58-a4cd-70cfe21b29e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0be61d08-895d-4b05-bafc-721419687468" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9b5dfa4a-77c2-4abb-b4bb-3b808e06bd4f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0be61d08-895d-4b05-bafc-721419687468" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_4dfe0e74-7933-4806-84b6-bfa97a4a488f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0be61d08-895d-4b05-bafc-721419687468" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_4dfe0e74-7933-4806-84b6-bfa97a4a488f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsConversionInPeriod_ef15f2d2-6c31-47c2-896a-523e51e0d537" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsConversionInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0be61d08-895d-4b05-bafc-721419687468" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsConversionInPeriod_ef15f2d2-6c31-47c2-896a-523e51e0d537" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_79eda02d-ae7a-4604-9094-50970838000d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0be61d08-895d-4b05-bafc-721419687468" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_79eda02d-ae7a-4604-9094-50970838000d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_bbcedb98-0f17-4c44-bbba-fb37c5e98ce7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0be61d08-895d-4b05-bafc-721419687468" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_bbcedb98-0f17-4c44-bbba-fb37c5e98ce7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCancelledInPeriod_53595806-3c35-494d-b24e-ac676ea81709" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCancelledInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_0be61d08-895d-4b05-bafc-721419687468" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCancelledInPeriod_53595806-3c35-494d-b24e-ac676ea81709" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_66b91bb0-688e-4bc5-9bd6-e55466612d2b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d1462e3a-00f5-4f0a-8a04-d18045d6c2a3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9b5dfa4a-77c2-4abb-b4bb-3b808e06bd4f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d1462e3a-00f5-4f0a-8a04-d18045d6c2a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_183cc45a-f9e7-48af-bce0-aba8f2ef97be" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d1462e3a-00f5-4f0a-8a04-d18045d6c2a3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_183cc45a-f9e7-48af-bce0-aba8f2ef97be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsConversionsWeightedAverageGrantDateFairValue_8f242a78-cefa-46ca-b5e3-f7b8795bf8df" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsConversionsWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d1462e3a-00f5-4f0a-8a04-d18045d6c2a3" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsConversionsWeightedAverageGrantDateFairValue_8f242a78-cefa-46ca-b5e3-f7b8795bf8df" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_7850f9f6-5136-424f-adf2-f9f4be6a7d01" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d1462e3a-00f5-4f0a-8a04-d18045d6c2a3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_7850f9f6-5136-424f-adf2-f9f4be6a7d01" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_b37d8739-c5da-4a5c-bf6d-eb96e3845ea3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d1462e3a-00f5-4f0a-8a04-d18045d6c2a3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_b37d8739-c5da-4a5c-bf6d-eb96e3845ea3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCancelledInPeriodWeightedAverageGrantDateFairValue_74fd3fa4-0c37-48da-84e5-f2aaa99b4fb1" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCancelledInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_d1462e3a-00f5-4f0a-8a04-d18045d6c2a3" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCancelledInPeriodWeightedAverageGrantDateFairValue_74fd3fa4-0c37-48da-84e5-f2aaa99b4fb1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_29852ffe-a6b1-4fbb-baa7-4f5031b8d1d6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/STOCKBASEDCOMPENSATION2024EmployeeStockPurchasePlanDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#STOCKBASEDCOMPENSATION2024EmployeeStockPurchasePlanDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/STOCKBASEDCOMPENSATION2024EmployeeStockPurchasePlanDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b050fbf7-8ac2-44bb-81ea-1729a507676f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_b0b8dcba-3a9c-40e8-b5cc-03e816413ee2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b050fbf7-8ac2-44bb-81ea-1729a507676f" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_b0b8dcba-3a9c-40e8-b5cc-03e816413ee2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_b1fcaf43-2366-4816-ad20-3e77c788f30b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_b0b8dcba-3a9c-40e8-b5cc-03e816413ee2" xlink:to="loc_us-gaap_PlanNameAxis_b1fcaf43-2366-4816-ad20-3e77c788f30b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_b1fcaf43-2366-4816-ad20-3e77c788f30b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_b1fcaf43-2366-4816-ad20-3e77c788f30b" xlink:to="loc_us-gaap_PlanNameDomain_b1fcaf43-2366-4816-ad20-3e77c788f30b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_4afb786e-de5b-4af7-bcdb-e6e273e1ef74" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_b1fcaf43-2366-4816-ad20-3e77c788f30b" xlink:to="loc_us-gaap_PlanNameDomain_4afb786e-de5b-4af7-bcdb-e6e273e1ef74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_TheGRAILInc.2024EmployeeStockPurchasePlanMember_89f32926-16c7-47eb-a8e7-ee09479cd5ee" xlink:href="gral-20240630.xsd#gral_TheGRAILInc.2024EmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4afb786e-de5b-4af7-bcdb-e6e273e1ef74" xlink:to="loc_gral_TheGRAILInc.2024EmployeeStockPurchasePlanMember_89f32926-16c7-47eb-a8e7-ee09479cd5ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_8c3f86a9-1e5b-447e-8324-bcdb37bbba80" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_b0b8dcba-3a9c-40e8-b5cc-03e816413ee2" xlink:to="loc_us-gaap_AwardTypeAxis_8c3f86a9-1e5b-447e-8324-bcdb37bbba80" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8c3f86a9-1e5b-447e-8324-bcdb37bbba80_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_8c3f86a9-1e5b-447e-8324-bcdb37bbba80" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8c3f86a9-1e5b-447e-8324-bcdb37bbba80_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_61ef3209-9bfa-4078-8fba-9168d0c61a64" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_8c3f86a9-1e5b-447e-8324-bcdb37bbba80" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_61ef3209-9bfa-4078-8fba-9168d0c61a64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_c8d67a41-a852-43b8-947e-dd457ccd91c9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_61ef3209-9bfa-4078-8fba-9168d0c61a64" xlink:to="loc_us-gaap_EmployeeStockMember_c8d67a41-a852-43b8-947e-dd457ccd91c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_9163db8f-8079-4c08-ad26-fee4c2422c3b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b050fbf7-8ac2-44bb-81ea-1729a507676f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_9163db8f-8079-4c08-ad26-fee4c2422c3b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationByShareBasedPaymentAwardPercentageOfOutstandingStockAnnualIncrease_21641457-8546-4eae-acbd-da9eb197f115" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationByShareBasedPaymentAwardPercentageOfOutstandingStockAnnualIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b050fbf7-8ac2-44bb-81ea-1729a507676f" xlink:to="loc_gral_ShareBasedCompensationByShareBasedPaymentAwardPercentageOfOutstandingStockAnnualIncrease_21641457-8546-4eae-acbd-da9eb197f115" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_4cd3b430-4a0b-405b-80a1-1fc129cc8379" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b050fbf7-8ac2-44bb-81ea-1729a507676f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_4cd3b430-4a0b-405b-80a1-1fc129cc8379" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/STOCKBASEDCOMPENSATION2024TransitionIncentiveAwardDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#STOCKBASEDCOMPENSATION2024TransitionIncentiveAwardDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/STOCKBASEDCOMPENSATION2024TransitionIncentiveAwardDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e37f0982-ecea-40fc-a02a-59075067b6d8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d7684f2e-cf94-430a-8c75-21c5a8409492" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e37f0982-ecea-40fc-a02a-59075067b6d8" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d7684f2e-cf94-430a-8c75-21c5a8409492" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_e53163b2-e77e-43a0-805b-0fb6661c4e2f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d7684f2e-cf94-430a-8c75-21c5a8409492" xlink:to="loc_us-gaap_AwardTypeAxis_e53163b2-e77e-43a0-805b-0fb6661c4e2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e53163b2-e77e-43a0-805b-0fb6661c4e2f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_e53163b2-e77e-43a0-805b-0fb6661c4e2f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e53163b2-e77e-43a0-805b-0fb6661c4e2f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f02bac62-873c-445c-92a0-0923a133b272" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_e53163b2-e77e-43a0-805b-0fb6661c4e2f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f02bac62-873c-445c-92a0-0923a133b272" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_68746ade-8fa7-4bb1-944a-ccd4b230596e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f02bac62-873c-445c-92a0-0923a133b272" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_68746ade-8fa7-4bb1-944a-ccd4b230596e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_TransitionIncentiveAwardMember_5ba63377-4a0e-4573-a988-2c7617b6ad34" xlink:href="gral-20240630.xsd#gral_TransitionIncentiveAwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f02bac62-873c-445c-92a0-0923a133b272" xlink:to="loc_gral_TransitionIncentiveAwardMember_5ba63377-4a0e-4573-a988-2c7617b6ad34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_84774b7d-cb6d-4ee6-b4ae-db24b643483f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_d7684f2e-cf94-430a-8c75-21c5a8409492" xlink:to="loc_us-gaap_PlanNameAxis_84774b7d-cb6d-4ee6-b4ae-db24b643483f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_84774b7d-cb6d-4ee6-b4ae-db24b643483f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_84774b7d-cb6d-4ee6-b4ae-db24b643483f" xlink:to="loc_us-gaap_PlanNameDomain_84774b7d-cb6d-4ee6-b4ae-db24b643483f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_5791dbf5-2b0d-4a8a-b741-b8865faad623" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_84774b7d-cb6d-4ee6-b4ae-db24b643483f" xlink:to="loc_us-gaap_PlanNameDomain_5791dbf5-2b0d-4a8a-b741-b8865faad623" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_A2024TransitionIncentiveAwardPlanMember_739d1bcd-8f1b-4dcd-8788-eb1759178cea" xlink:href="gral-20240630.xsd#gral_A2024TransitionIncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_5791dbf5-2b0d-4a8a-b741-b8865faad623" xlink:to="loc_gral_A2024TransitionIncentiveAwardPlanMember_739d1bcd-8f1b-4dcd-8788-eb1759178cea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue_f5ef4e90-67d7-429e-9427-ce1ad7a73d9c" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e37f0982-ecea-40fc-a02a-59075067b6d8" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue_f5ef4e90-67d7-429e-9427-ce1ad7a73d9c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_141ef229-3f3b-4876-bdb6-9903ffa85d00" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e37f0982-ecea-40fc-a02a-59075067b6d8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_141ef229-3f3b-4876-bdb6-9903ffa85d00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementMeasurementInputNumberOfTradingDaysAfterSpinOff_bfa72a00-29bc-42d9-afba-5da87080d00c" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementMeasurementInputNumberOfTradingDaysAfterSpinOff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e37f0982-ecea-40fc-a02a-59075067b6d8" xlink:to="loc_gral_ShareBasedCompensationArrangementMeasurementInputNumberOfTradingDaysAfterSpinOff_bfa72a00-29bc-42d9-afba-5da87080d00c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalPaidInCapitalReclassificationOfLiabilityClassifiedAwardsDueToSpinoff_bcc9ab69-5f3b-4424-a616-b06730830f40" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalPaidInCapitalReclassificationOfLiabilityClassifiedAwardsDueToSpinoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e37f0982-ecea-40fc-a02a-59075067b6d8" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalPaidInCapitalReclassificationOfLiabilityClassifiedAwardsDueToSpinoff_bcc9ab69-5f3b-4424-a616-b06730830f40" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_e5ede9bb-6211-4d48-8245-79dbabb53c21" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e37f0982-ecea-40fc-a02a-59075067b6d8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_e5ede9bb-6211-4d48-8245-79dbabb53c21" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/STOCKBASEDCOMPENSATION2021CashBasedEquityAwardsNarrativeDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#STOCKBASEDCOMPENSATION2021CashBasedEquityAwardsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/STOCKBASEDCOMPENSATION2021CashBasedEquityAwardsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7da48fe6-d380-4cf1-b38f-ecb62c25cc83" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4c46986a-a5ad-4ee8-9841-c2b2d2dda44c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7da48fe6-d380-4cf1-b38f-ecb62c25cc83" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4c46986a-a5ad-4ee8-9841-c2b2d2dda44c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_e2dd5e78-241d-4438-bd96-2bf2140dc47c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4c46986a-a5ad-4ee8-9841-c2b2d2dda44c" xlink:to="loc_us-gaap_AwardTypeAxis_e2dd5e78-241d-4438-bd96-2bf2140dc47c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e2dd5e78-241d-4438-bd96-2bf2140dc47c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_e2dd5e78-241d-4438-bd96-2bf2140dc47c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e2dd5e78-241d-4438-bd96-2bf2140dc47c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0896db51-058b-4c1c-8c5d-3a6cd3ac2778" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_e2dd5e78-241d-4438-bd96-2bf2140dc47c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0896db51-058b-4c1c-8c5d-3a6cd3ac2778" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_5fa2bf89-2e58-4f0a-8116-36ab12efcb39" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0896db51-058b-4c1c-8c5d-3a6cd3ac2778" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_5fa2bf89-2e58-4f0a-8116-36ab12efcb39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_98bb9a1c-a3f3-42e3-9d14-c2509415cc65" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4c46986a-a5ad-4ee8-9841-c2b2d2dda44c" xlink:to="loc_us-gaap_PlanNameAxis_98bb9a1c-a3f3-42e3-9d14-c2509415cc65" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_98bb9a1c-a3f3-42e3-9d14-c2509415cc65_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_98bb9a1c-a3f3-42e3-9d14-c2509415cc65" xlink:to="loc_us-gaap_PlanNameDomain_98bb9a1c-a3f3-42e3-9d14-c2509415cc65_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_108896f8-3ea3-4389-a8f7-3f9ce541420d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_98bb9a1c-a3f3-42e3-9d14-c2509415cc65" xlink:to="loc_us-gaap_PlanNameDomain_108896f8-3ea3-4389-a8f7-3f9ce541420d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_A2021CashBasedEquityAwardsProgramMember_815d067d-44ff-4ec1-9869-a71576daea1f" xlink:href="gral-20240630.xsd#gral_A2021CashBasedEquityAwardsProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_108896f8-3ea3-4389-a8f7-3f9ce541420d" xlink:to="loc_gral_A2021CashBasedEquityAwardsProgramMember_815d067d-44ff-4ec1-9869-a71576daea1f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_11d0239a-164a-4ffc-870f-a8517ed59b0b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7da48fe6-d380-4cf1-b38f-ecb62c25cc83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_11d0239a-164a-4ffc-870f-a8517ed59b0b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_4b8e692b-2382-4d4b-99d2-125308c087cc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7da48fe6-d380-4cf1-b38f-ecb62c25cc83" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_4b8e692b-2382-4d4b-99d2-125308c087cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementMeasurementInputNumberOfTradingDaysAfterSpinOff_49d74232-ecc3-43fe-b11b-ca4b2065337c" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementMeasurementInputNumberOfTradingDaysAfterSpinOff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7da48fe6-d380-4cf1-b38f-ecb62c25cc83" xlink:to="loc_gral_ShareBasedCompensationArrangementMeasurementInputNumberOfTradingDaysAfterSpinOff_49d74232-ecc3-43fe-b11b-ca4b2065337c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationNumberOfEmployeesAffected_9d3dd3e3-4a11-47b6-bb0f-d28c9b8ed0af" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationNumberOfEmployeesAffected"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7da48fe6-d380-4cf1-b38f-ecb62c25cc83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationNumberOfEmployeesAffected_9d3dd3e3-4a11-47b6-bb0f-d28c9b8ed0af" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_AdditionalPaidInCapitalReclassificationOfLiabilityClassifiedEquityAwardsFairValue_60ee9caf-25a5-41ee-bf96-41b3285babe0" xlink:href="gral-20240630.xsd#gral_AdditionalPaidInCapitalReclassificationOfLiabilityClassifiedEquityAwardsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7da48fe6-d380-4cf1-b38f-ecb62c25cc83" xlink:to="loc_gral_AdditionalPaidInCapitalReclassificationOfLiabilityClassifiedEquityAwardsFairValue_60ee9caf-25a5-41ee-bf96-41b3285babe0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_21f0ecfe-c5b6-427a-a427-e807bb3f8a9b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7da48fe6-d380-4cf1-b38f-ecb62c25cc83" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_21f0ecfe-c5b6-427a-a427-e807bb3f8a9b" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/STOCKBASEDCOMPENSATIONCashBasedEquityAwardsActivityDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#STOCKBASEDCOMPENSATIONCashBasedEquityAwardsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/STOCKBASEDCOMPENSATIONCashBasedEquityAwardsActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1f1e5b3f-7f94-42c9-b237-ab056f1b9dbc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_cb259df1-eadf-41c7-9c98-f1b878b096ac" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1f1e5b3f-7f94-42c9-b237-ab056f1b9dbc" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_cb259df1-eadf-41c7-9c98-f1b878b096ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_7bc44ff5-4131-4ba8-b164-178a46d32bed" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_cb259df1-eadf-41c7-9c98-f1b878b096ac" xlink:to="loc_us-gaap_PlanNameAxis_7bc44ff5-4131-4ba8-b164-178a46d32bed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_7bc44ff5-4131-4ba8-b164-178a46d32bed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_7bc44ff5-4131-4ba8-b164-178a46d32bed" xlink:to="loc_us-gaap_PlanNameDomain_7bc44ff5-4131-4ba8-b164-178a46d32bed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_b6b605bf-573b-4136-9a0b-41240392bd6c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_7bc44ff5-4131-4ba8-b164-178a46d32bed" xlink:to="loc_us-gaap_PlanNameDomain_b6b605bf-573b-4136-9a0b-41240392bd6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_A2021CashBasedEquityAwardsProgramMember_3fb26fde-5b94-4266-b378-18a96bb71242" xlink:href="gral-20240630.xsd#gral_A2021CashBasedEquityAwardsProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_b6b605bf-573b-4136-9a0b-41240392bd6c" xlink:to="loc_gral_A2021CashBasedEquityAwardsProgramMember_3fb26fde-5b94-4266-b378-18a96bb71242" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_4f29bef5-b1e6-45b2-a6c7-06ff5328241c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_cb259df1-eadf-41c7-9c98-f1b878b096ac" xlink:to="loc_us-gaap_AwardTypeAxis_4f29bef5-b1e6-45b2-a6c7-06ff5328241c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4f29bef5-b1e6-45b2-a6c7-06ff5328241c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_4f29bef5-b1e6-45b2-a6c7-06ff5328241c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4f29bef5-b1e6-45b2-a6c7-06ff5328241c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6f6ad36e-591d-4dde-b96e-d4892f89b262" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_4f29bef5-b1e6-45b2-a6c7-06ff5328241c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6f6ad36e-591d-4dde-b96e-d4892f89b262" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_6c7161a3-8a92-4795-9310-aa3dfa12d09c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6f6ad36e-591d-4dde-b96e-d4892f89b262" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_6c7161a3-8a92-4795-9310-aa3dfa12d09c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e3fdd6d9-2376-4ad2-a384-6bd712a55d58" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1f1e5b3f-7f94-42c9-b237-ab056f1b9dbc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e3fdd6d9-2376-4ad2-a384-6bd712a55d58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedFairValue_1f8c1989-3c54-4796-8159-438982f459e7" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e3fdd6d9-2376-4ad2-a384-6bd712a55d58" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedFairValue_1f8c1989-3c54-4796-8159-438982f459e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue_8506436b-ac48-4573-8115-d335c9ddc852" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e3fdd6d9-2376-4ad2-a384-6bd712a55d58" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue_8506436b-ac48-4573-8115-d335c9ddc852" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCancelledInPeriodFairValue_df82911c-7654-4dd1-8184-881098a5db2c" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCancelledInPeriodFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e3fdd6d9-2376-4ad2-a384-6bd712a55d58" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsCancelledInPeriodFairValue_df82911c-7654-4dd1-8184-881098a5db2c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesPaid_939d7e6e-5364-4e13-a4aa-c1e8b08b2085" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e3fdd6d9-2376-4ad2-a384-6bd712a55d58" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesPaid_939d7e6e-5364-4e13-a4aa-c1e8b08b2085" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesIncreaseDecreaseInFairValue_b0897583-a55b-43f7-afe2-82a84a357346" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesIncreaseDecreaseInFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e3fdd6d9-2376-4ad2-a384-6bd712a55d58" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesIncreaseDecreaseInFairValue_b0897583-a55b-43f7-afe2-82a84a357346" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedPaymentArrangementByShareBasedPaymentAwardConversionOfLiabilityClassifiedEquityAwardsAmount_b9adf730-4e81-44b5-b6c4-aadf82ea5287" xlink:href="gral-20240630.xsd#gral_ShareBasedPaymentArrangementByShareBasedPaymentAwardConversionOfLiabilityClassifiedEquityAwardsAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e3fdd6d9-2376-4ad2-a384-6bd712a55d58" xlink:to="loc_gral_ShareBasedPaymentArrangementByShareBasedPaymentAwardConversionOfLiabilityClassifiedEquityAwardsAmount_b9adf730-4e81-44b5-b6c4-aadf82ea5287" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedFairValue_b3f88f26-eced-4e03-9682-99fe4befe50b" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/STOCKBASEDCOMPENSATIONPerformanceBasedAwardDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#STOCKBASEDCOMPENSATIONPerformanceBasedAwardDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/STOCKBASEDCOMPENSATIONPerformanceBasedAwardDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b0b3fe1a-d519-4100-8d42-0d3a0c159bd2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_81cf8a09-f48c-431b-94bf-ad2a51740e9c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b0b3fe1a-d519-4100-8d42-0d3a0c159bd2" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_81cf8a09-f48c-431b-94bf-ad2a51740e9c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_a71c4df3-03ca-4dd8-8361-5ade23d6e1b2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_81cf8a09-f48c-431b-94bf-ad2a51740e9c" xlink:to="loc_us-gaap_AwardTypeAxis_a71c4df3-03ca-4dd8-8361-5ade23d6e1b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a71c4df3-03ca-4dd8-8361-5ade23d6e1b2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_a71c4df3-03ca-4dd8-8361-5ade23d6e1b2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a71c4df3-03ca-4dd8-8361-5ade23d6e1b2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_67f8f8a3-2b56-4c84-a0b9-dfa15c1d0fc6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_a71c4df3-03ca-4dd8-8361-5ade23d6e1b2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_67f8f8a3-2b56-4c84-a0b9-dfa15c1d0fc6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_e1716072-a7af-4f18-adc3-2c2a2a26ce9f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_67f8f8a3-2b56-4c84-a0b9-dfa15c1d0fc6" xlink:to="loc_us-gaap_PerformanceSharesMember_e1716072-a7af-4f18-adc3-2c2a2a26ce9f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusAxis_7d680a2d-99bc-46d5-a4db-d20d95d8ccf6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GranteeStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_81cf8a09-f48c-431b-94bf-ad2a51740e9c" xlink:to="loc_us-gaap_GranteeStatusAxis_7d680a2d-99bc-46d5-a4db-d20d95d8ccf6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_7d680a2d-99bc-46d5-a4db-d20d95d8ccf6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GranteeStatusAxis_7d680a2d-99bc-46d5-a4db-d20d95d8ccf6" xlink:to="loc_us-gaap_GranteeStatusDomain_7d680a2d-99bc-46d5-a4db-d20d95d8ccf6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_bd208fa8-9b0d-4908-b125-508ef554882c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GranteeStatusAxis_7d680a2d-99bc-46d5-a4db-d20d95d8ccf6" xlink:to="loc_us-gaap_GranteeStatusDomain_bd208fa8-9b0d-4908-b125-508ef554882c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_FormerEmployeeMember_8cf2eb77-74fe-4e09-90bb-e941e65253b5" xlink:href="gral-20240630.xsd#gral_FormerEmployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_bd208fa8-9b0d-4908-b125-508ef554882c" xlink:to="loc_gral_FormerEmployeeMember_8cf2eb77-74fe-4e09-90bb-e941e65253b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_b4a7a077-fec7-4cf3-9360-d76f9f9b4552" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_81cf8a09-f48c-431b-94bf-ad2a51740e9c" xlink:to="loc_us-gaap_VestingAxis_b4a7a077-fec7-4cf3-9360-d76f9f9b4552" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_b4a7a077-fec7-4cf3-9360-d76f9f9b4552_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_b4a7a077-fec7-4cf3-9360-d76f9f9b4552" xlink:to="loc_us-gaap_VestingDomain_b4a7a077-fec7-4cf3-9360-d76f9f9b4552_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_eb039bc5-5ca4-46a7-8575-b63218ab690c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_b4a7a077-fec7-4cf3-9360-d76f9f9b4552" xlink:to="loc_us-gaap_VestingDomain_eb039bc5-5ca4-46a7-8575-b63218ab690c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_4122422a-2c54-4b6f-822e-b1f2e353c019" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_eb039bc5-5ca4-46a7-8575-b63218ab690c" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_4122422a-2c54-4b6f-822e-b1f2e353c019" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_d5d9a3a6-4316-486d-9783-e2ae3da5866f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_eb039bc5-5ca4-46a7-8575-b63218ab690c" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_d5d9a3a6-4316-486d-9783-e2ae3da5866f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_cba444df-3249-4db3-805e-151dae6f19ae" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationAwardTrancheThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_eb039bc5-5ca4-46a7-8575-b63218ab690c" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_cba444df-3249-4db3-805e-151dae6f19ae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedPaymentArrangementTrancheFourMember_55090c8e-ebfb-44f4-9031-052a0d70d0fe" xlink:href="gral-20240630.xsd#gral_ShareBasedPaymentArrangementTrancheFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_eb039bc5-5ca4-46a7-8575-b63218ab690c" xlink:to="loc_gral_ShareBasedPaymentArrangementTrancheFourMember_55090c8e-ebfb-44f4-9031-052a0d70d0fe" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregatePotentialFairValue_dd826788-3498-4de5-a31d-fa3c6af1d0b8" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregatePotentialFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b0b3fe1a-d519-4100-8d42-0d3a0c159bd2" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregatePotentialFairValue_dd826788-3498-4de5-a31d-fa3c6af1d0b8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceAwardNetRevenueThreshold_eeb688cf-7269-4014-9d08-045bc3c34363" xlink:href="gral-20240630.xsd#gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceAwardNetRevenueThreshold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b0b3fe1a-d519-4100-8d42-0d3a0c159bd2" xlink:to="loc_gral_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceAwardNetRevenueThreshold_eeb688cf-7269-4014-9d08-045bc3c34363" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/STOCKBASEDCOMPENSATIONPerformanceOptionsDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#STOCKBASEDCOMPENSATIONPerformanceOptionsDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/STOCKBASEDCOMPENSATIONPerformanceOptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_aa7bf343-4d58-499f-8507-87ab0197edb6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8947d232-8347-4825-821a-29e0786b0ecb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_aa7bf343-4d58-499f-8507-87ab0197edb6" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8947d232-8347-4825-821a-29e0786b0ecb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_0117681d-301d-4290-957e-cc050a08c25a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8947d232-8347-4825-821a-29e0786b0ecb" xlink:to="loc_us-gaap_AwardTypeAxis_0117681d-301d-4290-957e-cc050a08c25a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0117681d-301d-4290-957e-cc050a08c25a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_0117681d-301d-4290-957e-cc050a08c25a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0117681d-301d-4290-957e-cc050a08c25a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2b680bd9-7fc5-4859-a754-6ae53c12d25c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_0117681d-301d-4290-957e-cc050a08c25a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2b680bd9-7fc5-4859-a754-6ae53c12d25c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_88f8e852-bb0b-4d27-b98a-436d23ae9146" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2b680bd9-7fc5-4859-a754-6ae53c12d25c" xlink:to="loc_us-gaap_EmployeeStockOptionMember_88f8e852-bb0b-4d27-b98a-436d23ae9146" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_ea6cf8fc-e2f8-4f19-aa81-7bf0126bdd10" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_aa7bf343-4d58-499f-8507-87ab0197edb6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_ea6cf8fc-e2f8-4f19-aa81-7bf0126bdd10" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_247e34e3-634f-4213-af17-c04c31200cbc" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_aa7bf343-4d58-499f-8507-87ab0197edb6" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_247e34e3-634f-4213-af17-c04c31200cbc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_02fa7277-fb8c-42ca-9585-958df357ca84" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_aa7bf343-4d58-499f-8507-87ab0197edb6" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_02fa7277-fb8c-42ca-9585-958df357ca84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_fbdc6f57-48d1-4825-8357-0088683a2d29" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_aa7bf343-4d58-499f-8507-87ab0197edb6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_fbdc6f57-48d1-4825-8357-0088683a2d29" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_d44a3b62-efc7-4346-9fb0-f012280a1e0c" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_aa7bf343-4d58-499f-8507-87ab0197edb6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_d44a3b62-efc7-4346-9fb0-f012280a1e0c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_4b3a9de7-5558-4285-b0d1-b66a5b718be2" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_aa7bf343-4d58-499f-8507-87ab0197edb6" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_4b3a9de7-5558-4285-b0d1-b66a5b718be2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_eb09d8a4-878d-4c55-b2b1-7f59a0ba6d17" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_aa7bf343-4d58-499f-8507-87ab0197edb6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_eb09d8a4-878d-4c55-b2b1-7f59a0ba6d17" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSNarrativeDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#RELATEDPARTYTRANSACTIONSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_6a9cb806-4f12-4135-9ed2-d84b19f474a5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_fdab811b-df29-4070-9c87-9f979364fe7b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_6a9cb806-4f12-4135-9ed2-d84b19f474a5" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_fdab811b-df29-4070-9c87-9f979364fe7b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_e83ca010-d000-436f-ba5a-d0e26c5d3d50" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_fdab811b-df29-4070-9c87-9f979364fe7b" xlink:to="loc_srt_OwnershipAxis_e83ca010-d000-436f-ba5a-d0e26c5d3d50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_e83ca010-d000-436f-ba5a-d0e26c5d3d50_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_e83ca010-d000-436f-ba5a-d0e26c5d3d50" xlink:to="loc_srt_OwnershipDomain_e83ca010-d000-436f-ba5a-d0e26c5d3d50_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_146b6e79-6b97-4b45-bf36-cb9910926c3c" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_e83ca010-d000-436f-ba5a-d0e26c5d3d50" xlink:to="loc_srt_OwnershipDomain_146b6e79-6b97-4b45-bf36-cb9910926c3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_GrailInc.Member_1f0725c5-ed94-4dd5-9273-7392877f52f7" xlink:href="gral-20240630.xsd#gral_GrailInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_146b6e79-6b97-4b45-bf36-cb9910926c3c" xlink:to="loc_gral_GrailInc.Member_1f0725c5-ed94-4dd5-9273-7392877f52f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_860d25c5-f90f-4d99-9dad-6aa5749e49a1" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_fdab811b-df29-4070-9c87-9f979364fe7b" xlink:to="loc_dei_LegalEntityAxis_860d25c5-f90f-4d99-9dad-6aa5749e49a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_860d25c5-f90f-4d99-9dad-6aa5749e49a1_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_860d25c5-f90f-4d99-9dad-6aa5749e49a1" xlink:to="loc_dei_EntityDomain_860d25c5-f90f-4d99-9dad-6aa5749e49a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_89a7509d-7813-4525-96c9-100ea4906681" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_860d25c5-f90f-4d99-9dad-6aa5749e49a1" xlink:to="loc_dei_EntityDomain_89a7509d-7813-4525-96c9-100ea4906681" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_8dfacfd7-18fa-4fd8-af43-569f8112e9cc" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_89a7509d-7813-4525-96c9-100ea4906681" xlink:to="loc_gral_IlluminaInc.Member_8dfacfd7-18fa-4fd8-af43-569f8112e9cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityOwnershipPercentageExcludingConsolidatedEntityAndEquityMethodInvestee_ce5a7f17-38b1-4e10-8656-d0bade4e2c58" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityOwnershipPercentageExcludingConsolidatedEntityAndEquityMethodInvestee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_6a9cb806-4f12-4135-9ed2-d84b19f474a5" xlink:to="loc_us-gaap_EquityOwnershipPercentageExcludingConsolidatedEntityAndEquityMethodInvestee_ce5a7f17-38b1-4e10-8656-d0bade4e2c58" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartytransactionsbybalancesheetgroupingDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#RELATEDPARTYTRANSACTIONSRelatedpartytransactionsbybalancesheetgroupingDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartytransactionsbybalancesheetgroupingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_536654b4-5669-4eea-881d-9edf7ee13fdd" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_7218f850-f8f9-4ccf-bbde-321dddc80216" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_536654b4-5669-4eea-881d-9edf7ee13fdd" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_7218f850-f8f9-4ccf-bbde-321dddc80216" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_ee764594-ffb1-42b9-87b1-4f43f6d518e7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_7218f850-f8f9-4ccf-bbde-321dddc80216" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_ee764594-ffb1-42b9-87b1-4f43f6d518e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_ee764594-ffb1-42b9-87b1-4f43f6d518e7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_ee764594-ffb1-42b9-87b1-4f43f6d518e7" xlink:to="loc_us-gaap_RelatedPartyDomain_ee764594-ffb1-42b9-87b1-4f43f6d518e7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_ba7433e2-89aa-477e-aef1-9753f82d7ed8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_ee764594-ffb1-42b9-87b1-4f43f6d518e7" xlink:to="loc_us-gaap_RelatedPartyDomain_ba7433e2-89aa-477e-aef1-9753f82d7ed8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_baf7a3ee-07a7-4739-9869-5f88a8dc8190" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_ba7433e2-89aa-477e-aef1-9753f82d7ed8" xlink:to="loc_us-gaap_RelatedPartyMember_baf7a3ee-07a7-4739-9869-5f88a8dc8190" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_799837ad-614d-4e02-acd8-d481d91dc3e2" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_7218f850-f8f9-4ccf-bbde-321dddc80216" xlink:to="loc_srt_CounterpartyNameAxis_799837ad-614d-4e02-acd8-d481d91dc3e2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_799837ad-614d-4e02-acd8-d481d91dc3e2_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_799837ad-614d-4e02-acd8-d481d91dc3e2" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_799837ad-614d-4e02-acd8-d481d91dc3e2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_379053b4-6b01-4710-8adb-bfb1c42b1674" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_799837ad-614d-4e02-acd8-d481d91dc3e2" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_379053b4-6b01-4710-8adb-bfb1c42b1674" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_f951c06f-a5dc-4f9d-993e-93e3d93edfaa" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_379053b4-6b01-4710-8adb-bfb1c42b1674" xlink:to="loc_gral_IlluminaInc.Member_f951c06f-a5dc-4f9d-993e-93e3d93edfaa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_1cbe0234-6069-429c-a7b3-3dd46a5edfce" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_536654b4-5669-4eea-881d-9edf7ee13fdd" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_1cbe0234-6069-429c-a7b3-3dd46a5edfce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_SuppliesCurrent_b8d2f870-2f3c-4082-8f95-3b7a00fe0359" xlink:href="gral-20240630.xsd#gral_SuppliesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_536654b4-5669-4eea-881d-9edf7ee13fdd" xlink:to="loc_gral_SuppliesCurrent_b8d2f870-2f3c-4082-8f95-3b7a00fe0359" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_2bcf8e90-5d0f-4f88-b1c3-f4fcc555ebfe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_536654b4-5669-4eea-881d-9edf7ee13fdd" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_2bcf8e90-5d0f-4f88-b1c3-f4fcc555ebfe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_4d76e349-671f-4070-ab39-8ca85082315b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_536654b4-5669-4eea-881d-9edf7ee13fdd" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_4d76e349-671f-4070-ab39-8ca85082315b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_feaf8261-ec62-4aba-ae51-562bdac5676a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_536654b4-5669-4eea-881d-9edf7ee13fdd" xlink:to="loc_us-gaap_AccountsPayableCurrent_feaf8261-ec62-4aba-ae51-562bdac5676a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_267856ee-e5c5-4c3f-b8a0-253adb5591a1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_536654b4-5669-4eea-881d-9edf7ee13fdd" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_267856ee-e5c5-4c3f-b8a0-253adb5591a1" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartydisclosuresbyincomestatementgroupingDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#RELATEDPARTYTRANSACTIONSRelatedpartydisclosuresbyincomestatementgroupingDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartydisclosuresbyincomestatementgroupingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_32eb06f2-f24a-49a8-a319-fc0f9ad785fb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_5ec1b77e-7bf4-49a1-890d-9cd9584380e3" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_32eb06f2-f24a-49a8-a319-fc0f9ad785fb" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_5ec1b77e-7bf4-49a1-890d-9cd9584380e3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_cd1367b8-6395-4324-913e-b7276d56b220" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_5ec1b77e-7bf4-49a1-890d-9cd9584380e3" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_cd1367b8-6395-4324-913e-b7276d56b220" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_cd1367b8-6395-4324-913e-b7276d56b220_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_cd1367b8-6395-4324-913e-b7276d56b220" xlink:to="loc_us-gaap_RelatedPartyDomain_cd1367b8-6395-4324-913e-b7276d56b220_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_410d47e3-d00a-46ab-b676-9b5887b16fad" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_cd1367b8-6395-4324-913e-b7276d56b220" xlink:to="loc_us-gaap_RelatedPartyDomain_410d47e3-d00a-46ab-b676-9b5887b16fad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_0eda264f-afb8-414b-91eb-3bc5c5bdfb65" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_410d47e3-d00a-46ab-b676-9b5887b16fad" xlink:to="loc_us-gaap_RelatedPartyMember_0eda264f-afb8-414b-91eb-3bc5c5bdfb65" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_42bc52ee-d104-4587-9ad4-f52aa3a7376e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_5ec1b77e-7bf4-49a1-890d-9cd9584380e3" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_42bc52ee-d104-4587-9ad4-f52aa3a7376e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_42bc52ee-d104-4587-9ad4-f52aa3a7376e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_42bc52ee-d104-4587-9ad4-f52aa3a7376e" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_42bc52ee-d104-4587-9ad4-f52aa3a7376e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_3e749e06-3fc6-4004-baeb-e64fca3fbc16" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_42bc52ee-d104-4587-9ad4-f52aa3a7376e" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_3e749e06-3fc6-4004-baeb-e64fca3fbc16" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_CostOfScreeningRevenueMember_e2e8b9a7-e77e-4a44-b269-b4f90aacc8b7" xlink:href="gral-20240630.xsd#gral_CostOfScreeningRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_3e749e06-3fc6-4004-baeb-e64fca3fbc16" xlink:to="loc_gral_CostOfScreeningRevenueMember_e2e8b9a7-e77e-4a44-b269-b4f90aacc8b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_CostOfDevelopmentServicesRevenueMember_8da1ce12-e063-4310-8893-c73af1614d44" xlink:href="gral-20240630.xsd#gral_CostOfDevelopmentServicesRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_3e749e06-3fc6-4004-baeb-e64fca3fbc16" xlink:to="loc_gral_CostOfDevelopmentServicesRevenueMember_8da1ce12-e063-4310-8893-c73af1614d44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_31fce882-d7be-4b41-a47f-badf383f83fd" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_5ec1b77e-7bf4-49a1-890d-9cd9584380e3" xlink:to="loc_srt_CounterpartyNameAxis_31fce882-d7be-4b41-a47f-badf383f83fd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_31fce882-d7be-4b41-a47f-badf383f83fd_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_31fce882-d7be-4b41-a47f-badf383f83fd" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_31fce882-d7be-4b41-a47f-badf383f83fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_16acfe59-d7c9-46b9-a95e-b5863d74fd30" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_31fce882-d7be-4b41-a47f-badf383f83fd" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_16acfe59-d7c9-46b9-a95e-b5863d74fd30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_dc86509b-c695-41c1-b22d-ae02da2bcef0" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_16acfe59-d7c9-46b9-a95e-b5863d74fd30" xlink:to="loc_gral_IlluminaInc.Member_dc86509b-c695-41c1-b22d-ae02da2bcef0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_2723d9f6-c317-4cf5-8a3a-93224cfc90f3" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_5ec1b77e-7bf4-49a1-890d-9cd9584380e3" xlink:to="loc_srt_ProductOrServiceAxis_2723d9f6-c317-4cf5-8a3a-93224cfc90f3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_2723d9f6-c317-4cf5-8a3a-93224cfc90f3_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_2723d9f6-c317-4cf5-8a3a-93224cfc90f3" xlink:to="loc_srt_ProductsAndServicesDomain_2723d9f6-c317-4cf5-8a3a-93224cfc90f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_69acce4d-397d-4fc4-afc4-8ec4f1d9b46d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_2723d9f6-c317-4cf5-8a3a-93224cfc90f3" xlink:to="loc_srt_ProductsAndServicesDomain_69acce4d-397d-4fc4-afc4-8ec4f1d9b46d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ScreeningRevenueMember_b37d7f4d-a03b-4006-8fa4-1689a4f084eb" xlink:href="gral-20240630.xsd#gral_ScreeningRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_69acce4d-397d-4fc4-afc4-8ec4f1d9b46d" xlink:to="loc_gral_ScreeningRevenueMember_b37d7f4d-a03b-4006-8fa4-1689a4f084eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_7ea1c998-e14b-4352-a0fc-39757eb46be5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_32eb06f2-f24a-49a8-a319-fc0f9ad785fb" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_7ea1c998-e14b-4352-a0fc-39757eb46be5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_aaadad3c-6b14-44db-84f8-e36ff0f36a1e" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_32eb06f2-f24a-49a8-a319-fc0f9ad785fb" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_aaadad3c-6b14-44db-84f8-e36ff0f36a1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_8f002369-7879-4d8b-ae8d-7e07c43b70b7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_32eb06f2-f24a-49a8-a319-fc0f9ad785fb" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_8f002369-7879-4d8b-ae8d-7e07c43b70b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_559c2606-eb96-4d8b-9715-1d1358c41cfe" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_32eb06f2-f24a-49a8-a319-fc0f9ad785fb" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_559c2606-eb96-4d8b-9715-1d1358c41cfe" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartycontributionsbymemberDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#RELATEDPARTYTRANSACTIONSRelatedpartycontributionsbymemberDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartycontributionsbymemberDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_3e324df6-a5b4-4ad3-9efe-153a88ebb53a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_dd08e320-bdec-46a5-9153-efe2e897fac4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e324df6-a5b4-4ad3-9efe-153a88ebb53a" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_dd08e320-bdec-46a5-9153-efe2e897fac4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_64f65a8b-9542-489d-abfc-cf2d68f4dc68" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_dd08e320-bdec-46a5-9153-efe2e897fac4" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_64f65a8b-9542-489d-abfc-cf2d68f4dc68" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_64f65a8b-9542-489d-abfc-cf2d68f4dc68_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_64f65a8b-9542-489d-abfc-cf2d68f4dc68" xlink:to="loc_us-gaap_RelatedPartyDomain_64f65a8b-9542-489d-abfc-cf2d68f4dc68_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_81482124-dc40-4023-acdf-869c1d09b120" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_64f65a8b-9542-489d-abfc-cf2d68f4dc68" xlink:to="loc_us-gaap_RelatedPartyDomain_81482124-dc40-4023-acdf-869c1d09b120" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_95d394d0-1931-4954-b674-675a534d7aeb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_81482124-dc40-4023-acdf-869c1d09b120" xlink:to="loc_us-gaap_RelatedPartyMember_95d394d0-1931-4954-b674-675a534d7aeb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_39458a65-5e85-41eb-8832-bea8080b7d92" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_dd08e320-bdec-46a5-9153-efe2e897fac4" xlink:to="loc_srt_CounterpartyNameAxis_39458a65-5e85-41eb-8832-bea8080b7d92" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_39458a65-5e85-41eb-8832-bea8080b7d92_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_39458a65-5e85-41eb-8832-bea8080b7d92" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_39458a65-5e85-41eb-8832-bea8080b7d92_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_741c1b4f-fd74-4e3c-ab9c-b0e3a800daae" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_39458a65-5e85-41eb-8832-bea8080b7d92" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_741c1b4f-fd74-4e3c-ab9c-b0e3a800daae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_b101e952-6315-4e89-bdb6-44ab9bba38fc" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_741c1b4f-fd74-4e3c-ab9c-b0e3a800daae" xlink:to="loc_gral_IlluminaInc.Member_b101e952-6315-4e89-bdb6-44ab9bba38fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromContributionsFromAffiliates_17e0de1b-0965-44c6-9fe3-885f1d942fc8" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProceedsFromContributionsFromAffiliates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e324df6-a5b4-4ad3-9efe-153a88ebb53a" xlink:to="loc_us-gaap_ProceedsFromContributionsFromAffiliates_17e0de1b-0965-44c6-9fe3-885f1d942fc8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_1aceb119-9f2b-46c0-8c8d-02d9ed002187" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e324df6-a5b4-4ad3-9efe-153a88ebb53a" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_1aceb119-9f2b-46c0-8c8d-02d9ed002187" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ProceedsFromContributionsFromAffiliatesNet_c853338d-3814-4571-bbda-0c3142a4a362" xlink:href="gral-20240630.xsd#gral_ProceedsFromContributionsFromAffiliatesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_3e324df6-a5b4-4ad3-9efe-153a88ebb53a" xlink:to="loc_gral_ProceedsFromContributionsFromAffiliatesNet_c853338d-3814-4571-bbda-0c3142a4a362" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartybalancesheettransactionsbyexecutiveofficerDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#RELATEDPARTYTRANSACTIONSRelatedpartybalancesheettransactionsbyexecutiveofficerDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartybalancesheettransactionsbyexecutiveofficerDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_e54c0c31-c689-47be-9017-e07fabcfcced" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_51abdec3-6629-415f-be8e-c764a1627ce5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_e54c0c31-c689-47be-9017-e07fabcfcced" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_51abdec3-6629-415f-be8e-c764a1627ce5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_dfe1ea56-2eb4-4e3e-a6c3-699fec344272" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_51abdec3-6629-415f-be8e-c764a1627ce5" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_dfe1ea56-2eb4-4e3e-a6c3-699fec344272" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_dfe1ea56-2eb4-4e3e-a6c3-699fec344272_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_dfe1ea56-2eb4-4e3e-a6c3-699fec344272" xlink:to="loc_us-gaap_RelatedPartyDomain_dfe1ea56-2eb4-4e3e-a6c3-699fec344272_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_6ae686f7-c9f1-4499-ad5e-851cf1ae4f3f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_dfe1ea56-2eb4-4e3e-a6c3-699fec344272" xlink:to="loc_us-gaap_RelatedPartyDomain_6ae686f7-c9f1-4499-ad5e-851cf1ae4f3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ChiefExecutiveOfficerMember_3e309c61-dad3-4ec0-9ae2-439d9655015b" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_6ae686f7-c9f1-4499-ad5e-851cf1ae4f3f" xlink:to="loc_srt_ChiefExecutiveOfficerMember_3e309c61-dad3-4ec0-9ae2-439d9655015b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_ffdcafd4-09fa-4b97-b65a-78ab92c3c6c7" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_51abdec3-6629-415f-be8e-c764a1627ce5" xlink:to="loc_srt_CounterpartyNameAxis_ffdcafd4-09fa-4b97-b65a-78ab92c3c6c7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_ffdcafd4-09fa-4b97-b65a-78ab92c3c6c7_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_ffdcafd4-09fa-4b97-b65a-78ab92c3c6c7" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_ffdcafd4-09fa-4b97-b65a-78ab92c3c6c7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_dc962c80-9d65-4444-8823-7b34b4726f27" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_ffdcafd4-09fa-4b97-b65a-78ab92c3c6c7" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_dc962c80-9d65-4444-8823-7b34b4726f27" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_TwistBioscienceMember_b49756c9-3c77-4bfe-9d31-04bf57fbd38c" xlink:href="gral-20240630.xsd#gral_TwistBioscienceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_dc962c80-9d65-4444-8823-7b34b4726f27" xlink:to="loc_gral_TwistBioscienceMember_b49756c9-3c77-4bfe-9d31-04bf57fbd38c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_SuppliesCurrent_a8fe9c3e-2a7a-41de-ac1a-0a46f2794129" xlink:href="gral-20240630.xsd#gral_SuppliesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_e54c0c31-c689-47be-9017-e07fabcfcced" xlink:to="loc_gral_SuppliesCurrent_a8fe9c3e-2a7a-41de-ac1a-0a46f2794129" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_455f13a6-e780-4602-b311-e16e7e3d4986" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_e54c0c31-c689-47be-9017-e07fabcfcced" xlink:to="loc_us-gaap_AccountsPayableCurrent_455f13a6-e780-4602-b311-e16e7e3d4986" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartyincomestatementtransactionsbyexecutiveofficerDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#RELATEDPARTYTRANSACTIONSRelatedpartyincomestatementtransactionsbyexecutiveofficerDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/RELATEDPARTYTRANSACTIONSRelatedpartyincomestatementtransactionsbyexecutiveofficerDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_81a523bf-5c0b-4428-b11d-ff341bbf6969" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_2cc78934-4ad6-4bb4-a075-e9328733dd86" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_81a523bf-5c0b-4428-b11d-ff341bbf6969" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_2cc78934-4ad6-4bb4-a075-e9328733dd86" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_35129f5b-01b3-4dc9-bf56-1032603b9afb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_2cc78934-4ad6-4bb4-a075-e9328733dd86" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_35129f5b-01b3-4dc9-bf56-1032603b9afb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_35129f5b-01b3-4dc9-bf56-1032603b9afb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_35129f5b-01b3-4dc9-bf56-1032603b9afb" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_35129f5b-01b3-4dc9-bf56-1032603b9afb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_31fd2dd4-4836-4522-b208-a88dac4704ef" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_35129f5b-01b3-4dc9-bf56-1032603b9afb" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_31fd2dd4-4836-4522-b208-a88dac4704ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_CostOfScreeningRevenueMember_65814f3d-eeba-4d8e-a31a-3362c95bb959" xlink:href="gral-20240630.xsd#gral_CostOfScreeningRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_31fd2dd4-4836-4522-b208-a88dac4704ef" xlink:to="loc_gral_CostOfScreeningRevenueMember_65814f3d-eeba-4d8e-a31a-3362c95bb959" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_CostOfDevelopmentServicesRevenueMember_50f2aaf0-3e24-4161-9683-d88a6343415c" xlink:href="gral-20240630.xsd#gral_CostOfDevelopmentServicesRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_31fd2dd4-4836-4522-b208-a88dac4704ef" xlink:to="loc_gral_CostOfDevelopmentServicesRevenueMember_50f2aaf0-3e24-4161-9683-d88a6343415c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_ad63ef25-9e1a-4909-8684-4a1708605d44" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_2cc78934-4ad6-4bb4-a075-e9328733dd86" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_ad63ef25-9e1a-4909-8684-4a1708605d44" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_ad63ef25-9e1a-4909-8684-4a1708605d44_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_ad63ef25-9e1a-4909-8684-4a1708605d44" xlink:to="loc_us-gaap_RelatedPartyDomain_ad63ef25-9e1a-4909-8684-4a1708605d44_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_c5dc86e4-950a-4618-86a8-31b696c2a315" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_ad63ef25-9e1a-4909-8684-4a1708605d44" xlink:to="loc_us-gaap_RelatedPartyDomain_c5dc86e4-950a-4618-86a8-31b696c2a315" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ChiefExecutiveOfficerMember_884b22f3-50bf-4c8a-9fd3-fba2c74667c9" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_ChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_c5dc86e4-950a-4618-86a8-31b696c2a315" xlink:to="loc_srt_ChiefExecutiveOfficerMember_884b22f3-50bf-4c8a-9fd3-fba2c74667c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_63bb666d-f11c-498d-abef-6760a7264a72" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_2cc78934-4ad6-4bb4-a075-e9328733dd86" xlink:to="loc_srt_CounterpartyNameAxis_63bb666d-f11c-498d-abef-6760a7264a72" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_63bb666d-f11c-498d-abef-6760a7264a72_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_63bb666d-f11c-498d-abef-6760a7264a72" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_63bb666d-f11c-498d-abef-6760a7264a72_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_9fb10d27-38ff-4333-82ba-210cf9033d34" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_63bb666d-f11c-498d-abef-6760a7264a72" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_9fb10d27-38ff-4333-82ba-210cf9033d34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_TwistBioscienceMember_7f5a364f-89f1-4efd-9edf-5462ba3e9df6" xlink:href="gral-20240630.xsd#gral_TwistBioscienceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_9fb10d27-38ff-4333-82ba-210cf9033d34" xlink:to="loc_gral_TwistBioscienceMember_7f5a364f-89f1-4efd-9edf-5462ba3e9df6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_39c63978-9b18-49d8-b451-a008d83073b6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_81a523bf-5c0b-4428-b11d-ff341bbf6969" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_39c63978-9b18-49d8-b451-a008d83073b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_1cd4f415-1a7f-45fa-b530-e0a9e320aeaf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_81a523bf-5c0b-4428-b11d-ff341bbf6969" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_1cd4f415-1a7f-45fa-b530-e0a9e320aeaf" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/TAXESDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#TAXESDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/TAXESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_4573e9a2-4919-4824-8d1c-396f908a5ee0" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_1fea6881-6661-4170-b94a-60949bd7055b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_OperatingLossCarryforwardsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4573e9a2-4919-4824-8d1c-396f908a5ee0" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_1fea6881-6661-4170-b94a-60949bd7055b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_5c3230b3-fa9c-4bdf-8357-4971336a02b5" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_1fea6881-6661-4170-b94a-60949bd7055b" xlink:to="loc_dei_LegalEntityAxis_5c3230b3-fa9c-4bdf-8357-4971336a02b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_5c3230b3-fa9c-4bdf-8357-4971336a02b5_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_5c3230b3-fa9c-4bdf-8357-4971336a02b5" xlink:to="loc_dei_EntityDomain_5c3230b3-fa9c-4bdf-8357-4971336a02b5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_890f1595-3d5b-43b2-921d-ebfec770557a" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_5c3230b3-fa9c-4bdf-8357-4971336a02b5" xlink:to="loc_dei_EntityDomain_890f1595-3d5b-43b2-921d-ebfec770557a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_06605d61-91fd-41f8-84e8-78d9cd2d26b1" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_890f1595-3d5b-43b2-921d-ebfec770557a" xlink:to="loc_gral_IlluminaInc.Member_06605d61-91fd-41f8-84e8-78d9cd2d26b1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_11c73d95-556d-4518-bac5-2ceff0a1158a" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_1fea6881-6661-4170-b94a-60949bd7055b" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_11c73d95-556d-4518-bac5-2ceff0a1158a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_11c73d95-556d-4518-bac5-2ceff0a1158a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_11c73d95-556d-4518-bac5-2ceff0a1158a" xlink:to="loc_us-gaap_EquityComponentDomain_11c73d95-556d-4518-bac5-2ceff0a1158a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_c139bef0-af63-4cc5-8c3f-70b40efbe2eb" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_11c73d95-556d-4518-bac5-2ceff0a1158a" xlink:to="loc_us-gaap_EquityComponentDomain_c139bef0-af63-4cc5-8c3f-70b40efbe2eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_047b28c4-55e0-42e9-83da-df5be2623eea" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_c139bef0-af63-4cc5-8c3f-70b40efbe2eb" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_047b28c4-55e0-42e9-83da-df5be2623eea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_f75ef8f4-2de5-4b9e-a2de-68b79c81b05b" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4573e9a2-4919-4824-8d1c-396f908a5ee0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_f75ef8f4-2de5-4b9e-a2de-68b79c81b05b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedResearchAndDevelopmentCosts_f6bc2831-6572-4dd3-aad9-2dca2406dfc4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedResearchAndDevelopmentCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4573e9a2-4919-4824-8d1c-396f908a5ee0" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedResearchAndDevelopmentCosts_f6bc2831-6572-4dd3-aad9-2dca2406dfc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_StockholdersEquityDecreaseForTaxLiabilityDueToSpinOffTransaction_bbf87708-343d-497d-9f2a-337d4eb05584" xlink:href="gral-20240630.xsd#gral_StockholdersEquityDecreaseForTaxLiabilityDueToSpinOffTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_4573e9a2-4919-4824-8d1c-396f908a5ee0" xlink:to="loc_gral_StockholdersEquityDecreaseForTaxLiabilityDueToSpinOffTransaction_bbf87708-343d-497d-9f2a-337d4eb05584" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/LEGALANDREGULATORYPROCEEDINGSDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#LEGALANDREGULATORYPROCEEDINGSDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/LEGALANDREGULATORYPROCEEDINGSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_503123a9-7c44-428b-9cde-481d42ad7de9" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_7b57ee4a-3026-4335-a1e0-31ee6db572a7" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_503123a9-7c44-428b-9cde-481d42ad7de9" xlink:to="loc_us-gaap_LossContingenciesTable_7b57ee4a-3026-4335-a1e0-31ee6db572a7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_52858bab-c2d4-4f71-817c-e2249689f2b6" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_7b57ee4a-3026-4335-a1e0-31ee6db572a7" xlink:to="loc_srt_OwnershipAxis_52858bab-c2d4-4f71-817c-e2249689f2b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_52858bab-c2d4-4f71-817c-e2249689f2b6_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_52858bab-c2d4-4f71-817c-e2249689f2b6" xlink:to="loc_srt_OwnershipDomain_52858bab-c2d4-4f71-817c-e2249689f2b6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_2ac54bad-f393-40d3-bff6-6aa44a96c225" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_52858bab-c2d4-4f71-817c-e2249689f2b6" xlink:to="loc_srt_OwnershipDomain_2ac54bad-f393-40d3-bff6-6aa44a96c225" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_GrailInc.Member_f1951ed1-906c-4078-b2d7-647ab6b2374f" xlink:href="gral-20240630.xsd#gral_GrailInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_2ac54bad-f393-40d3-bff6-6aa44a96c225" xlink:to="loc_gral_GrailInc.Member_f1951ed1-906c-4078-b2d7-647ab6b2374f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_7a3b7c4e-b1d4-4e9b-a013-ceee5c9c2333" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_7b57ee4a-3026-4335-a1e0-31ee6db572a7" xlink:to="loc_srt_CounterpartyNameAxis_7a3b7c4e-b1d4-4e9b-a013-ceee5c9c2333" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7a3b7c4e-b1d4-4e9b-a013-ceee5c9c2333_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_7a3b7c4e-b1d4-4e9b-a013-ceee5c9c2333" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7a3b7c4e-b1d4-4e9b-a013-ceee5c9c2333_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_ad717c6f-77d2-4997-9a0e-452f6c680be3" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_7a3b7c4e-b1d4-4e9b-a013-ceee5c9c2333" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_ad717c6f-77d2-4997-9a0e-452f6c680be3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_3df3f20d-a48a-412b-b9e9-f16aafd585eb" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_ad717c6f-77d2-4997-9a0e-452f6c680be3" xlink:to="loc_gral_IlluminaInc.Member_3df3f20d-a48a-412b-b9e9-f16aafd585eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_64928204-3069-447e-aa9c-080ec427ae3e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_7b57ee4a-3026-4335-a1e0-31ee6db572a7" xlink:to="loc_dei_LegalEntityAxis_64928204-3069-447e-aa9c-080ec427ae3e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_64928204-3069-447e-aa9c-080ec427ae3e_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_64928204-3069-447e-aa9c-080ec427ae3e" xlink:to="loc_dei_EntityDomain_64928204-3069-447e-aa9c-080ec427ae3e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_2958713b-9ef7-4548-a11f-7c97201bb29c" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_64928204-3069-447e-aa9c-080ec427ae3e" xlink:to="loc_dei_EntityDomain_2958713b-9ef7-4548-a11f-7c97201bb29c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_IlluminaInc.Member_9092d0c9-a2da-4d85-9265-5e51eaf93434" xlink:href="gral-20240630.xsd#gral_IlluminaInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_2958713b-9ef7-4548-a11f-7c97201bb29c" xlink:to="loc_gral_IlluminaInc.Member_9092d0c9-a2da-4d85-9265-5e51eaf93434" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityOwnershipPercentageExcludingConsolidatedEntityAndEquityMethodInvestee_7e6f0ae2-2516-4b15-9556-2873e651626f" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EquityOwnershipPercentageExcludingConsolidatedEntityAndEquityMethodInvestee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_503123a9-7c44-428b-9cde-481d42ad7de9" xlink:to="loc_us-gaap_EquityOwnershipPercentageExcludingConsolidatedEntityAndEquityMethodInvestee_7e6f0ae2-2516-4b15-9556-2873e651626f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromContributionsFromAffiliates_689bed81-fc4e-45bf-85ec-7147f4904bb4" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ProceedsFromContributionsFromAffiliates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_503123a9-7c44-428b-9cde-481d42ad7de9" xlink:to="loc_us-gaap_ProceedsFromContributionsFromAffiliates_689bed81-fc4e-45bf-85ec-7147f4904bb4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_ContributionFromAffiliateClawbackTerm_664ab4cd-e3ed-4f92-a222-d11125159c69" xlink:href="gral-20240630.xsd#gral_ContributionFromAffiliateClawbackTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_503123a9-7c44-428b-9cde-481d42ad7de9" xlink:to="loc_gral_ContributionFromAffiliateClawbackTerm_664ab4cd-e3ed-4f92-a222-d11125159c69" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.grail.com/role/SUBSEQUENTEVENTSDetails" xlink:type="simple" xlink:href="gral-20240630.xsd#SUBSEQUENTEVENTSDetails"/>
  <link:definitionLink xlink:role="http://www.grail.com/role/SUBSEQUENTEVENTSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_aa21d01f-a8ed-4578-8ae3-7bbc46b86329" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_14d42158-5f9d-4f52-92ea-2752283ba50d" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_aa21d01f-a8ed-4578-8ae3-7bbc46b86329" xlink:to="loc_us-gaap_SubsequentEventTable_14d42158-5f9d-4f52-92ea-2752283ba50d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_4813fdc9-f1bc-4fc7-9660-d9a1140cdaf1" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_14d42158-5f9d-4f52-92ea-2752283ba50d" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_4813fdc9-f1bc-4fc7-9660-d9a1140cdaf1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_4813fdc9-f1bc-4fc7-9660-d9a1140cdaf1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_4813fdc9-f1bc-4fc7-9660-d9a1140cdaf1" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_4813fdc9-f1bc-4fc7-9660-d9a1140cdaf1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_ff825c07-644a-4341-938d-61cb47ac02cf" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_4813fdc9-f1bc-4fc7-9660-d9a1140cdaf1" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_ff825c07-644a-4341-938d-61cb47ac02cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_28e60155-c6b9-4bcf-b126-0f3e5dca64e6" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_ff825c07-644a-4341-938d-61cb47ac02cf" xlink:to="loc_us-gaap_SubsequentEventMember_28e60155-c6b9-4bcf-b126-0f3e5dca64e6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_82f08015-b131-4eab-b344-31abd3407a42" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_14d42158-5f9d-4f52-92ea-2752283ba50d" xlink:to="loc_us-gaap_RestructuringPlanAxis_82f08015-b131-4eab-b344-31abd3407a42" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_82f08015-b131-4eab-b344-31abd3407a42_default" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_82f08015-b131-4eab-b344-31abd3407a42" xlink:to="loc_us-gaap_RestructuringPlanDomain_82f08015-b131-4eab-b344-31abd3407a42_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_261c4a33-d261-487b-b93e-c175d5211ae5" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_82f08015-b131-4eab-b344-31abd3407a42" xlink:to="loc_us-gaap_RestructuringPlanDomain_261c4a33-d261-487b-b93e-c175d5211ae5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_TwentyTwentyFourRestructuringPlanMember_dcc42b46-29aa-46e2-8323-421946682cea" xlink:href="gral-20240630.xsd#gral_TwentyTwentyFourRestructuringPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_261c4a33-d261-487b-b93e-c175d5211ae5" xlink:to="loc_gral_TwentyTwentyFourRestructuringPlanMember_dcc42b46-29aa-46e2-8323-421946682cea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_d40f74b6-b5d9-4f96-b0dd-0ad755fb0c69" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_14d42158-5f9d-4f52-92ea-2752283ba50d" xlink:to="loc_srt_RangeAxis_d40f74b6-b5d9-4f96-b0dd-0ad755fb0c69" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_d40f74b6-b5d9-4f96-b0dd-0ad755fb0c69_default" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_d40f74b6-b5d9-4f96-b0dd-0ad755fb0c69" xlink:to="loc_srt_RangeMember_d40f74b6-b5d9-4f96-b0dd-0ad755fb0c69_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_6611f9de-71c2-47e9-aef9-57d4da0c9f7a" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_d40f74b6-b5d9-4f96-b0dd-0ad755fb0c69" xlink:to="loc_srt_RangeMember_6611f9de-71c2-47e9-aef9-57d4da0c9f7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_39c71e6e-d720-40e3-ac9e-5574f7a86658" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_6611f9de-71c2-47e9-aef9-57d4da0c9f7a" xlink:to="loc_srt_MinimumMember_39c71e6e-d720-40e3-ac9e-5574f7a86658" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_43a20535-025e-494e-9524-894e1245485d" xlink:href="https://xbrl.fasb.org/srt/2024/elts/srt-2024.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_6611f9de-71c2-47e9-aef9-57d4da0c9f7a" xlink:to="loc_srt_MaximumMember_43a20535-025e-494e-9524-894e1245485d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_RestructuringAndRelatedCostExpectedNumberOfPositionsEliminatedPercentOfExistingAndPlannedAdditions_22e271be-1d54-4845-9d56-64b0ffc84f43" xlink:href="gral-20240630.xsd#gral_RestructuringAndRelatedCostExpectedNumberOfPositionsEliminatedPercentOfExistingAndPlannedAdditions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_aa21d01f-a8ed-4578-8ae3-7bbc46b86329" xlink:to="loc_gral_RestructuringAndRelatedCostExpectedNumberOfPositionsEliminatedPercentOfExistingAndPlannedAdditions_22e271be-1d54-4845-9d56-64b0ffc84f43" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostExpectedNumberOfPositionsEliminated_0d0f0758-ac02-479c-9c87-064558141e51" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringAndRelatedCostExpectedNumberOfPositionsEliminated"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_aa21d01f-a8ed-4578-8ae3-7bbc46b86329" xlink:to="loc_us-gaap_RestructuringAndRelatedCostExpectedNumberOfPositionsEliminated_0d0f0758-ac02-479c-9c87-064558141e51" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_RestructuringAndRelatedCostExpectedNumberOfPositionsEliminatedPercentOfExistingWorkforce_7c239c76-a70c-44ee-9053-7c6a89612dd8" xlink:href="gral-20240630.xsd#gral_RestructuringAndRelatedCostExpectedNumberOfPositionsEliminatedPercentOfExistingWorkforce"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_aa21d01f-a8ed-4578-8ae3-7bbc46b86329" xlink:to="loc_gral_RestructuringAndRelatedCostExpectedNumberOfPositionsEliminatedPercentOfExistingWorkforce_7c239c76-a70c-44ee-9053-7c6a89612dd8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_3fd41221-49ea-4497-b2fd-fb13bd189979" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestructuringAndRelatedCostExpectedCost1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_aa21d01f-a8ed-4578-8ae3-7bbc46b86329" xlink:to="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_3fd41221-49ea-4497-b2fd-fb13bd189979" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/AwardTypeOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2024/ecd-2024.xsd#AwardTypeOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/AwardTypeOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_EmployeeStockMember" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gral_TransitionIncentiveAwardMember_93c2a927-9da5-4348-8d7d-066d1f74366a" xlink:href="gral-20240630.xsd#gral_TransitionIncentiveAwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_gral_TransitionIncentiveAwardMember_93c2a927-9da5-4348-8d7d-066d1f74366a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaap_PerformanceSharesMember" xlink:type="arc" order="6"/>
  </link:definitionLink>
</link:linkbase>
