Index: clang/lib/Driver/ToolChains/MSVC.cpp
===================================================================
--- clang/lib/Driver/ToolChains/MSVC.cpp
+++ clang/lib/Driver/ToolChains/MSVC.cpp
@@ -1609,4 +1609,12 @@
   if (DriverArgs.hasArg(options::OPT_fno_rtti, options::OPT_frtti,
                         /*Default=*/false))
     CC1Args.push_back("-D_HAS_STATIC_RTTI=0");
+
+  // Starting with VS 2022 17.1, MSVC predefines the below macro to inform
+  // users of the execution character set defined at compile time.
+  // The value given is the Windows Code Page Identifier:
+  // https://docs.microsoft.com/en-us/windows/win32/intl/code-page-identifiers
+  //
+  // Clang currently only supports UTF-8, so we'll use 65001
+  CC1Args.push_back("-D_MSVC_EXECUTION_CHARACTER_SET=65001");
 }
Index: clang/test/Driver/cl-execution-character-set.c
===================================================================
--- /dev/null
+++ clang/test/Driver/cl-execution-character-set.c
@@ -0,0 +1,14 @@
+// Note: %s must be preceded by --, otherwise it may be interpreted as a
+// command-line option, e.g. on Mac where %s is commonly under /Users.
+
+// Only UTF-8 is currently supported, which has the Windows Code
+// Page Identifier 65001
+
+// Check this works with the clang-cl driver as well as with the GNU clang driver
+// RUN: %clang_cl -### -- %s 2>&1 | FileCheck -check-prefix=CHECK-MSVC %s
+// RUN: %clang --target=x86_64-pc-windows-msvc -### %s 2>&1 | FileCheck -check-prefix=CHECK-MSVC %s
+// CHECK-MSVC: -D_MSVC_EXECUTION_CHARACTER_SET=65001
+
+// It shouldn't be defined for any non MSVC targets
+// RUN: %clang --target=x86_64-pc-windows-gnu -### %s 2>&1 | FileCheck -check-prefix=CHECK-NON-MSVC %s
+// CHECK-NON-MSVC-NOT: -D_MSVC_EXECUTION_CHARACTER_SET