diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -110,6 +110,9 @@
 
 Modified Compiler Flags
 -----------------------
+- ``-fms-extensions`` now defines the ``_MSC_EXTENSIONS`` macro regardless of
+  whether or not the ``-fms-compatibility`` flag is set and regardless of the
+  target triple.
 
 Removed Compiler Flags
 -------------------------
diff --git a/clang/lib/Basic/Targets/OSTargets.cpp b/clang/lib/Basic/Targets/OSTargets.cpp
--- a/clang/lib/Basic/Targets/OSTargets.cpp
+++ b/clang/lib/Basic/Targets/OSTargets.cpp
@@ -226,16 +226,6 @@
     }
   }
 
-  if (Opts.MicrosoftExt) {
-    Builder.defineMacro("_MSC_EXTENSIONS");
-
-    if (Opts.CPlusPlus11) {
-      Builder.defineMacro("_RVALUE_REFERENCES_V2_SUPPORTED");
-      Builder.defineMacro("_RVALUE_REFERENCES_SUPPORTED");
-      Builder.defineMacro("_NATIVE_NULLPTR_SUPPORTED");
-    }
-  }
-
   if (!Opts.MSVolatile)
     Builder.defineMacro("_ISO_VOLATILE");
 
diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -926,11 +926,19 @@
     Builder.defineMacro("__private_extern__", "extern");
 
   if (LangOpts.MicrosoftExt) {
+    Builder.defineMacro("_MSC_EXTENSIONS");
+
     if (LangOpts.WChar) {
       // wchar_t supported as a keyword.
       Builder.defineMacro("_WCHAR_T_DEFINED");
       Builder.defineMacro("_NATIVE_WCHAR_T_DEFINED");
     }
+
+    if (LangOpts.CPlusPlus11) {
+      Builder.defineMacro("_RVALUE_REFERENCES_V2_SUPPORTED");
+      Builder.defineMacro("_RVALUE_REFERENCES_SUPPORTED");
+      Builder.defineMacro("_NATIVE_NULLPTR_SUPPORTED");
+    }
   }
 
   // Macros to help identify the narrow and wide character sets
diff --git a/clang/test/Preprocessor/predefined-win-macros.c b/clang/test/Preprocessor/predefined-win-macros.c
--- a/clang/test/Preprocessor/predefined-win-macros.c
+++ b/clang/test/Preprocessor/predefined-win-macros.c
@@ -120,6 +120,11 @@
 // CHECK-AMD64-MINGW: #define _WIN32 1
 // CHECK-AMD64-MINGW: #define _WIN64 1
 
+// RUN: %clang_cc1 -triple x86_64-windows-gnu %s -E -dM -o - -fms-extensions \
+// RUN:   | FileCheck -match-full-lines %s --check-prefix=CHECK-AMD64-MINGW-MSE
+
+// CHECK-AMD64-MINGW-MSE: #define _MSC_EXTENSIONS 1
+
 // RUN: %clang_cc1 -triple aarch64-windows-gnu %s -E -dM -o - \
 // RUN:   | FileCheck -match-full-lines %s --check-prefix=CHECK-ARM64-MINGW