diff --git a/docs/release-notes/.FSharp.Compiler.Service/9.0.200.md b/docs/release-notes/.FSharp.Compiler.Service/9.0.200.md
index 5ef93105656..d681f5aa1b7 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/9.0.200.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/9.0.200.md
@@ -1,5 +1,6 @@
 ### Fixed
 
+* Ensure `frameworkTcImportsCache` mutations are threadsafe. ([PR #17795](https://github.com/dotnet/fsharp/pull/17795))
 
 ### Added
 
diff --git a/src/Compiler/Service/IncrementalBuild.fs b/src/Compiler/Service/IncrementalBuild.fs
index b7560b222c2..6b7f3c4d24d 100644
--- a/src/Compiler/Service/IncrementalBuild.fs
+++ b/src/Compiler/Service/IncrementalBuild.fs
@@ -507,10 +507,12 @@ type FrameworkImportsCache(size) =
     let frameworkTcImportsCache = AgedLookup<AnyCallerThreadToken, FrameworkImportsCacheKey, GraphNode<TcGlobals * TcImports>>(size, areSimilar=(fun (x, y) -> x = y))
 
     /// Reduce the size of the cache in low-memory scenarios
-    member _.Downsize() = frameworkTcImportsCache.Resize(AnyCallerThread, newKeepStrongly=0)
+    member _.Downsize() = lock gate <| fun () ->
+        frameworkTcImportsCache.Resize(AnyCallerThread, newKeepStrongly=0)
 
     /// Clear the cache
-    member _.Clear() = frameworkTcImportsCache.Clear AnyCallerThread
+    member _.Clear() = lock gate <| fun () ->
+        frameworkTcImportsCache.Clear AnyCallerThread
 
     /// This function strips the "System" assemblies from the tcConfig and returns a age-cached TcImports for them.
     member _.GetNode(tcConfig: TcConfig, frameworkDLLs: AssemblyResolution list, nonFrameworkResolutions: AssemblyResolution list) =