diff --git a/src/libraries/System.Private.CoreLib/src/System/IO/Stream.cs b/src/libraries/System.Private.CoreLib/src/System/IO/Stream.cs
index 4a4c00efd30137..f8ac78c9bdac6e 100644
--- a/src/libraries/System.Private.CoreLib/src/System/IO/Stream.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/IO/Stream.cs
@@ -222,6 +222,9 @@ internal Task<int> BeginReadInternal(
             }
             else
             {
+#if FEATURE_SINGLE_THREADED
+                if (OperatingSystem.IsBrowser()) throw new PlatformNotSupportedException();
+#endif
 #pragma warning disable CA1416 // Validate platform compatibility, issue: https://github.com/dotnet/runtime/issues/44543
                 semaphore.Wait();
 #pragma warning restore CA1416
@@ -496,6 +499,9 @@ internal Task BeginWriteInternal(
             }
             else
             {
+#if FEATURE_SINGLE_THREADED
+                if (OperatingSystem.IsBrowser()) throw new PlatformNotSupportedException();
+#endif
 #pragma warning disable CA1416 // Validate platform compatibility, issue: https://github.com/dotnet/runtime/issues/44543
                 semaphore.Wait(); // synchronously wait here
 #pragma warning restore CA1416