diff --git a/clang/lib/Basic/Targets/AArch64.cpp b/clang/lib/Basic/Targets/AArch64.cpp
index 1569b5e04b770a..c8d243a8fb7aea 100644
--- a/clang/lib/Basic/Targets/AArch64.cpp
+++ b/clang/lib/Basic/Targets/AArch64.cpp
@@ -1543,10 +1543,13 @@ WindowsARM64TargetInfo::getBuiltinVaListKind() const {
 TargetInfo::CallingConvCheckResult
 WindowsARM64TargetInfo::checkCallingConvention(CallingConv CC) const {
   switch (CC) {
+  case CC_X86VectorCall:
+    if (getTriple().isWindowsArm64EC())
+      return CCCR_OK;
+    return CCCR_Ignore;
   case CC_X86StdCall:
   case CC_X86ThisCall:
   case CC_X86FastCall:
-  case CC_X86VectorCall:
     return CCCR_Ignore;
   case CC_C:
   case CC_OpenCLKernel:
diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index f12765b826935b..3f5463a9a70e9d 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -5591,6 +5591,12 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
                              /*AttrOnCallSite=*/true,
                              /*IsThunk=*/false);
 
+  if (CallingConv == llvm::CallingConv::X86_VectorCall &&
+      getTarget().getTriple().isWindowsArm64EC()) {
+    CGM.Error(Loc, "__vectorcall calling convention is not currently "
+                   "supported");
+  }
+
   if (const FunctionDecl *FD = dyn_cast_or_null<FunctionDecl>(CurFuncDecl)) {
     if (FD->hasAttr<StrictFPAttr>())
       // All calls within a strictfp function are marked strictfp
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 00b3bfcaa0bc25..75519be8bba052 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -2087,6 +2087,14 @@ void CodeGenModule::SetLLVMFunctionAttributes(GlobalDecl GD,
   llvm::AttributeList PAL;
   ConstructAttributeList(F->getName(), Info, GD, PAL, CallingConv,
                          /*AttrOnCallSite=*/false, IsThunk);
+  if (CallingConv == llvm::CallingConv::X86_VectorCall &&
+      getTarget().getTriple().isWindowsArm64EC()) {
+    SourceLocation Loc;
+    if (const Decl *D = GD.getDecl())
+      Loc = D->getLocation();
+
+    Error(Loc, "__vectorcall calling convention is not currently supported");
+  }
   F->setAttributes(PAL);
   F->setCallingConv(static_cast<llvm::CallingConv::ID>(CallingConv));
 }
diff --git a/clang/test/CodeGenCXX/arm64ec-vectorcall.cpp b/clang/test/CodeGenCXX/arm64ec-vectorcall.cpp
new file mode 100644
index 00000000000000..73d2d63835917c
--- /dev/null
+++ b/clang/test/CodeGenCXX/arm64ec-vectorcall.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -triple arm64ec-windows-msvc -emit-llvm -o - %s -verify
+
+// ARM64EC doesn't support generating __vectorcall calls... but __vectorcall
+// function types need to be distinct from __cdecl function types to support
+// compiling the STL. Make sure we only diagnose constructs that actually
+// require generating code.
+void __vectorcall f1();
+void f2(void __vectorcall p()) {}
+void f2(void p()) {}
+void __vectorcall (*f3)();
+void __vectorcall f4(); // expected-error {{__vectorcall}}
+void __vectorcall f5() { // expected-error {{__vectorcall}}
+  f4(); // expected-error{{__vectorcall}}
+}