Index: clang/include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -660,6 +660,10 @@
   "-fjmc works only for ELF; option ignored">,
   InGroup<OptionIgnored>;
 
+def warn_target_override_arm64ec : Warning<
+  "/arm64EC has been overridden by specified target:%0; option ignored">,
+  InGroup<OptionIgnored>;
+
 def err_drv_target_variant_invalid : Error<
   "unsupported '%0' value '%1'; use 'ios-macabi' instead">;
 
Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -6695,6 +6695,8 @@
 def _SLASH_QIntel_jcc_erratum : CLFlag<"QIntel-jcc-erratum">,
   HelpText<"Align branches within 32-byte boundaries to mitigate the performance impact of the Intel JCC erratum.">,
   Alias<mbranches_within_32B_boundaries>;
+def _SLASH_arm64EC : CLFlag<"arm64EC">,
+  HelpText<"Set build target to arm64ec">;
 
 // Non-aliases:
 
Index: clang/lib/Driver/Driver.cpp
===================================================================
--- clang/lib/Driver/Driver.cpp
+++ clang/lib/Driver/Driver.cpp
@@ -1252,6 +1252,8 @@
     T.setVendor(llvm::Triple::PC);
     T.setEnvironment(llvm::Triple::MSVC);
     T.setObjectFormat(llvm::Triple::COFF);
+    if (Args.hasArg(options::OPT__SLASH_arm64EC))
+      T.setArch(llvm::Triple::aarch64, llvm::Triple::AArch64SubArch_arm64ec);
     TargetTriple = T.str();
   } else if (IsDXCMode()) {
     // Build TargetTriple from target_profile option for clang-dxc.
@@ -1376,6 +1378,15 @@
   const ToolChain &TC = getToolChain(
       *UArgs, computeTargetTriple(*this, TargetTriple, *UArgs));
 
+  // Report warning when arm64EC option is overridden by specified target
+  if (TC.getTriple().getArch() != llvm::Triple::aarch64 ||
+      TC.getTriple().getSubArch() != llvm::Triple::AArch64SubArch_arm64ec) {
+    if (UArgs->hasArg(options::OPT__SLASH_arm64EC)) {
+      getDiags().Report(clang::diag::warn_target_override_arm64ec)
+          << TC.getTriple().str();
+    }
+  }
+
   // The compilation takes ownership of Args.
   Compilation *C = new Compilation(*this, TC, UArgs.release(), TranslatedArgs,
                                    ContainsError);
Index: clang/test/Driver/cl-options.c
===================================================================
--- clang/test/Driver/cl-options.c
+++ clang/test/Driver/cl-options.c
@@ -783,4 +783,10 @@
 // EXTERNAL_W0: "-Wno-system-headers"
 // EXTERNAL_Wn: "-Wsystem-headers"
 
+// RUN: %clang_cl -vctoolsdir "" /arm64EC /c -### -- %s 2>&1 | FileCheck %s --check-prefix ARM64EC
+// ARM64EC: "-triple" "arm64ec-pc-windows-msvc19.20.0"
+
+// RUN: %clang_cl -vctoolsdir "" /arm64EC /c -target x86_64-pc-windows-msvc  -### -- %s 2>&1 | FileCheck %s --check-prefix ARM64EC_OVERRIDE
+// ARM64EC_OVERRIDE: warning: /arm64EC has been overridden by specified target:x86_64-pc-windows-msvc; option ignored
+
 void f(void) { }