diff --git a/eng/RuntimeIdentifier.props b/eng/RuntimeIdentifier.props
index ea53b9e2d4e923..e2f854402c3364 100644
--- a/eng/RuntimeIdentifier.props
+++ b/eng/RuntimeIdentifier.props
@@ -69,6 +69,24 @@
     <TargetsWindows Condition="'$(TargetOS)' == 'windows'">true</TargetsWindows>
     <TargetsUnix Condition="'$(TargetsOpenBSD)' == 'true' or '$(TargetsFreeBSD)' == 'true' or '$(Targetsillumos)' == 'true' or '$(TargetsSolaris)' == 'true' or '$(TargetsHaiku)' == 'true' or '$(TargetsLinux)' == 'true' or '$(TargetsNetBSD)' == 'true' or '$(TargetsOSX)' == 'true' or '$(TargetsMacCatalyst)' == 'true' or '$(TargetstvOS)' == 'true' or '$(TargetsiOS)' == 'true' or '$(TargetsAndroid)' == 'true'">true</TargetsUnix>
   </PropertyGroup>
+  <!-- Values of the public OperatingSystem.IsXxx() checks for the TargetOS. These must match the
+       TARGET_* conditions in System.Private.CoreLib's OperatingSystem.cs. -->
+  <ItemGroup Label="TargetOperatingSystemChecks">
+    <TargetOperatingSystemCheck Include="IsAndroid" Value="$([MSBuild]::ValueOrDefault('$(TargetsAndroid)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsBrowser" Value="$([MSBuild]::ValueOrDefault('$(TargetsBrowser)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsFreeBSD" Value="$([MSBuild]::ValueOrDefault('$(TargetsFreeBSD)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsIOS" Value="true" Condition="'$(TargetsiOS)' == 'true' or '$(TargetsMacCatalyst)' == 'true'" />
+    <TargetOperatingSystemCheck Include="IsIOS" Value="false" Condition="'$(TargetsiOS)' != 'true' and '$(TargetsMacCatalyst)' != 'true'" />
+    <TargetOperatingSystemCheck Include="IsLinux" Value="true" Condition="'$(TargetsLinux)' == 'true' and '$(TargetsAndroid)' != 'true'" />
+    <TargetOperatingSystemCheck Include="IsLinux" Value="false" Condition="'$(TargetsLinux)' != 'true' or '$(TargetsAndroid)' == 'true'" />
+    <TargetOperatingSystemCheck Include="IsMacCatalyst" Value="$([MSBuild]::ValueOrDefault('$(TargetsMacCatalyst)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsMacOS" Value="$([MSBuild]::ValueOrDefault('$(TargetsOSX)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsOpenBSD" Value="$([MSBuild]::ValueOrDefault('$(TargetsOpenBSD)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsTvOS" Value="$([MSBuild]::ValueOrDefault('$(TargetstvOS)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsWasi" Value="$([MSBuild]::ValueOrDefault('$(TargetsWasi)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsWatchOS" Value="false" />
+    <TargetOperatingSystemCheck Include="IsWindows" Value="$([MSBuild]::ValueOrDefault('$(TargetsWindows)', 'false'))" />
+  </ItemGroup>
   <PropertyGroup>
     <_ImportedRuntimeIdentifierProps>true</_ImportedRuntimeIdentifierProps>
   </PropertyGroup>
diff --git a/eng/illink.targets b/eng/illink.targets
index 714ac859810554..7cdbe7ffb079e2 100644
--- a/eng/illink.targets
+++ b/eng/illink.targets
@@ -192,6 +192,70 @@
     </PropertyGroup>
   </Target>
 
+  <!-- OperatingSystem.IsXxx() are constants in the OS-specific System.Private.CoreLib, but the library build
+       resolves them from the System.Runtime reference assembly. Substitute their values for the OS being
+       built so that the library build trims code guarded by checks for other operating systems.
+       Not applied to packages: binaries shipped in them (e.g. a runtimes/unix asset) must remain valid on every
+       OS that their flavor covers. -->
+  <PropertyGroup>
+    <!-- The linux, ios and tvos flavors are also used by the TargetOS values that fall back to them:
+         android and openharmony (linux), maccatalyst and iossimulator (ios), tvossimulator (tvos). -->
+    <_OSSubstitutionsFlavorMatchesTargetOS Condition="'$(TargetOS)' != '' and
+                                                      ('$(TargetPlatformIdentifier)' == '$(TargetOS)' or
+                                                       ('$(TargetPlatformIdentifier)' == 'unix' and '$(TargetsUnix)' == 'true') or
+                                                       ('$(TargetPlatformIdentifier)' == 'linux' and '$(TargetsLinux)' == 'true') or
+                                                       ('$(TargetPlatformIdentifier)' == 'ios' and ('$(TargetsiOS)' == 'true' or '$(TargetsMacCatalyst)' == 'true')) or
+                                                       ('$(TargetPlatformIdentifier)' == 'tvos' and '$(TargetstvOS)' == 'true'))">true</_OSSubstitutionsFlavorMatchesTargetOS>
+    <!-- The OS whose values are substituted: the TargetOS for the flavors it uses, otherwise the OS of an OS-specific
+         flavor, so that a flavor produces the same binary regardless of the TargetOS the build was invoked for.
+         The unix flavor covers multiple operating systems, so it is only substituted when built for one of them. -->
+    <_OSSubstitutionsTargetOS Condition="'$(_OSSubstitutionsFlavorMatchesTargetOS)' == 'true'">$(TargetOS)</_OSSubstitutionsTargetOS>
+    <_OSSubstitutionsTargetOS Condition="'$(_OSSubstitutionsFlavorMatchesTargetOS)' != 'true' and '$(TargetPlatformIdentifier)' != 'unix'">$(TargetPlatformIdentifier.ToLowerInvariant())</_OSSubstitutionsTargetOS>
+    <_OSSubstitutionsApply Condition="'$(_OSSubstitutionsTargetOS)' != '' and
+                                      '$(IsPackable)' != 'true' and
+                                      '$(BuildAllConfigurations)' != 'true' and
+                                      '$(ILLinkSubstitutionsLibraryBuildXml)' == ''">true</_OSSubstitutionsApply>
+  </PropertyGroup>
+
+  <Target Name="_GetTargetOperatingSystemChecks"
+          Returns="@(TargetOperatingSystemCheck)" />
+
+  <Target Name="_GenerateOperatingSystemSubstitutions"
+          Condition="'$(_OSSubstitutionsApply)' == 'true' and '$(ILLinkTrimAssembly)' == 'true'"
+          BeforeTargets="ILLinkTrimAssembly">
+    <ItemGroup Condition="'$(_OSSubstitutionsTargetOS)' == '$(TargetOS)'">
+      <_OSSubstitutionsCheck Include="@(TargetOperatingSystemCheck)" />
+    </ItemGroup>
+
+    <!-- Evaluate the checks for the flavor's OS from the same definitions in RuntimeIdentifier.props. -->
+    <MSBuild Projects="$(MSBuildProjectFullPath)"
+             Targets="_GetTargetOperatingSystemChecks"
+             Properties="TargetOS=$(_OSSubstitutionsTargetOS);TargetFramework=$(TargetFramework)"
+             Condition="'$(_OSSubstitutionsTargetOS)' != '$(TargetOS)'">
+      <Output TaskParameter="TargetOutputs" ItemName="_OSSubstitutionsCheck" />
+    </MSBuild>
+
+    <PropertyGroup>
+      <ILLinkSubstitutionsLibraryBuildXml>$(IntermediateOutputPath)ILLink.Substitutions.OperatingSystem.LibraryBuild.xml</ILLinkSubstitutionsLibraryBuildXml>
+      <_OSSubstitutions><![CDATA[<linker>
+  <assembly fullname="System.Runtime">
+    <type fullname="System.OperatingSystem">
+@(_OSSubstitutionsCheck->'      <method signature="System.Boolean %(Identity)()" body="stub" value="%(Value)" />', '%0A')
+    </type>
+  </assembly>
+</linker>]]></_OSSubstitutions>
+    </PropertyGroup>
+
+    <WriteLinesToFile File="$(ILLinkSubstitutionsLibraryBuildXml)"
+                      Lines="$(_OSSubstitutions)"
+                      Overwrite="true"
+                      WriteOnlyWhenDifferent="true" />
+
+    <ItemGroup>
+      <FileWrites Include="$(ILLinkSubstitutionsLibraryBuildXml)" />
+    </ItemGroup>
+  </Target>
+
   <!-- Examines the "input assembly" for IL that is unreachable from public API and trims that,
        rewriting the assembly to an "output assembly" -->
   <UsingTask TaskName="ILLink"