diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -2450,6 +2450,12 @@
     return isX86VectorCallAggregateSmallEnough(NumMembers);
   }
 
+  ABIArgInfo classifyArgForArm64ECVarArg(QualType Ty) {
+    unsigned FreeSSERegs = 0;
+    return classify(Ty, FreeSSERegs, /*IsReturnType=*/false,
+                    /*IsVectorCall=*/false, /*IsRegCall=*/false);
+  }
+
 private:
   ABIArgInfo classify(QualType Ty, unsigned &FreeSSERegs, bool IsReturnType,
                       bool IsVectorCall, bool IsRegCall) const;
@@ -5735,6 +5741,13 @@
                                      unsigned CallingConvention) const {
   Ty = useFirstFieldIfTransparentUnion(Ty);
 
+  if (IsVariadic && getTarget().getTriple().isWindowsArm64EC()) {
+    // Arm64EC varargs functions use the x86_64 classification rules,
+    // not the AArch64 ABI rules.
+    WinX86_64ABIInfo Win64ABIInfo(CGT, X86AVXABILevel::None);
+    return Win64ABIInfo.classifyArgForArm64ECVarArg(Ty);
+  }
+
   // Handle illegal vector types here.
   if (isIllegalVectorType(Ty))
     return coerceIllegalVector(Ty);
diff --git a/clang/test/CodeGen/arm64ec.c b/clang/test/CodeGen/arm64ec.c
--- a/clang/test/CodeGen/arm64ec.c
+++ b/clang/test/CodeGen/arm64ec.c
@@ -2,6 +2,8 @@
 
 // CHECK: @g = alias void ([2 x float], [4 x float]), void ([2 x float], [4 x float])* @"#g"
 // CHECK: define dso_local void @"#g"
+// CHECK: call void (i64, ...) @f(i64 %{{.*}}, %struct.B* noundef %{{.*}})
 typedef struct { float x[2]; } A;
 typedef struct { float x[4]; } B;
-void g(A a, B b) { }
+void f(A a, ...);
+void g(A a, B b) { f(a, b); }
diff --git a/clang/test/CodeGenCXX/arm64ec.cpp b/clang/test/CodeGenCXX/arm64ec.cpp
--- a/clang/test/CodeGenCXX/arm64ec.cpp
+++ b/clang/test/CodeGenCXX/arm64ec.cpp
@@ -2,6 +2,8 @@
 
 // CHECK: @"?g@@YAXUA@@UB@@@Z" = alias void ([2 x float], [4 x float]), void ([2 x float], [4 x float])* @"?g@@$$hYAXUA@@UB@@@Z"
 // CHECK: define dso_local void @"?g@@$$hYAXUA@@UB@@@Z"
+// CHECK: call void (i64, ...) @"?f@@YAXUA@@ZZ"(i64 %{{.*}}, %struct.B* noundef %{{.*}})
 typedef struct { float x[2]; } A;
 typedef struct { float x[4]; } B;
-void g(A a, B b) { }
+void f(A a, ...);
+void g(A a, B b) { f(a, b); }