diff --git a/llvm/cmake/platforms/WinMsvc.cmake b/llvm/cmake/platforms/WinMsvc.cmake
--- a/llvm/cmake/platforms/WinMsvc.cmake
+++ b/llvm/cmake/platforms/WinMsvc.cmake
@@ -265,7 +265,8 @@
     --target=${TRIPLE_ARCH}-windows-msvc
     -fms-compatibility-version=19.20
     -vctoolsversion ${MSVC_VER}
-    -winsdkversion ${WINSDK_VER})
+    -winsdkversion ${WINSDK_VER}
+    -winsysroot ${LLVM_WINSYSROOT})
 
 if(case_sensitive_filesystem)
   # Ensure all sub-configures use the top-level VFS overlay instead of generating their own.