diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 2d3dc242c2f89..a9c7492c687be 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -1614,6 +1614,11 @@ ERROR(attr_objc_implementation_category_not_found,none,
 NOTE(attr_objc_implementation_fixit_remove_category_name,none,
      "remove arguments to implement the main '@interface' for this class",
      ())
+ERROR(attr_objc_implementation_no_conformance,none,
+      "'@_objcImplementation' extension cannot add conformance to %0; "
+      "add this conformance %select{with an ordinary extension|"
+      "in the Objective-C header}1",
+      (Type, bool))
 
 ERROR(member_of_objc_implementation_not_objc_or_final,none,
       "%0 %1 does not match any %0 declared in the headers for %2; did you use "
diff --git a/lib/Sema/TypeCheckDeclObjC.cpp b/lib/Sema/TypeCheckDeclObjC.cpp
index fd3bae37321f4..5f4ebdf8988cd 100644
--- a/lib/Sema/TypeCheckDeclObjC.cpp
+++ b/lib/Sema/TypeCheckDeclObjC.cpp
@@ -2906,6 +2906,18 @@ class ObjCImplementationChecker {
   {
     assert(!ext->hasClangNode() && "passed interface, not impl, to checker");
 
+    // Conformances are declared exclusively in the interface, so diagnose any
+    // in the implementation right away.
+    for (auto &inherited : ext->getInherited()) {
+      bool isImportedProtocol = false;
+      if (auto protoNominal = inherited.getType()->getAnyNominal())
+        isImportedProtocol = protoNominal->hasClangNode();
+
+      diagnose(inherited.getLoc(),
+               diag::attr_objc_implementation_no_conformance,
+               inherited.getType(), isImportedProtocol);
+    }
+
     // Did we actually match this extension to an interface? (In invalid code,
     // we might not have.)
     auto interfaceDecl = ext->getImplementedObjCDecl();
diff --git a/test/decl/ext/Inputs/objc_implementation.h b/test/decl/ext/Inputs/objc_implementation.h
index f6fc4856df29f..c4798088dd528 100644
--- a/test/decl/ext/Inputs/objc_implementation.h
+++ b/test/decl/ext/Inputs/objc_implementation.h
@@ -142,3 +142,6 @@
 struct ObjCStruct {
   int foo;
 };
+
+@protocol EmptyObjCProto
+@end
diff --git a/test/decl/ext/objc_implementation.swift b/test/decl/ext/objc_implementation.swift
index 06a266410bf83..b702b782fda10 100644
--- a/test/decl/ext/objc_implementation.swift
+++ b/test/decl/ext/objc_implementation.swift
@@ -1,13 +1,17 @@
 // RUN: %target-typecheck-verify-swift -import-objc-header %S/Inputs/objc_implementation.h
 // REQUIRES: objc_interop
 
-@_objcImplementation extension ObjCClass {
+protocol EmptySwiftProto {}
+
+@_objcImplementation extension ObjCClass: EmptySwiftProto, EmptyObjCProto {
   // expected-note@-1 {{previously implemented by extension here}}
   // expected-warning@-2 {{extension for main class interface should provide implementation for instance method 'method(fromHeader4:)'; this will become an error before '@_objcImplementation' is stabilized}}
   // expected-warning@-3 {{extension for main class interface should provide implementation for property 'propertyFromHeader9'; this will become an error before '@_objcImplementation' is stabilized}}
   // FIXME: give better diagnostic expected-warning@-4 {{extension for main class interface should provide implementation for property 'propertyFromHeader8'; this will become an error before '@_objcImplementation' is stabilized}}
   // FIXME: give better diagnostic expected-warning@-5 {{extension for main class interface should provide implementation for property 'propertyFromHeader7'; this will become an error before '@_objcImplementation' is stabilized}}
   // FIXME: give better diagnostic expected-warning@-6 {{extension for main class interface should provide implementation for instance method 'method(fromHeader3:)'; this will become an error before '@_objcImplementation' is stabilized}}
+  // expected-warning@-7 {{'@_objcImplementation' extension cannot add conformance to 'EmptySwiftProto'; add this conformance with an ordinary extension}}
+  // expected-warning@-8 {{'@_objcImplementation' extension cannot add conformance to 'EmptyObjCProto'; add this conformance in the Objective-C header}}
 
   func method(fromHeader1: CInt) {
     // OK, provides an implementation for the header's method.