diff --git a/include/swift/AST/DeclAttr.def b/include/swift/AST/DeclAttr.def
index ba2e8c28a28..f82320d0237 100644
--- a/include/swift/AST/DeclAttr.def
+++ b/include/swift/AST/DeclAttr.def
@@ -876,6 +876,11 @@ SIMPLE_DECL_ATTR(constInitialized, ConstInitialized,
   168)
 DECL_ATTR_FEATURE_REQUIREMENT(ConstInitialized, CompileTimeValues)
 
+SIMPLE_DECL_ATTR(extensible, Extensible,
+  OnEnum,
+  ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | ForbiddenInABIAttr,
+  169)
+
 SIMPLE_DECL_ATTR(concurrent, Concurrent,
   OnFunc | OnConstructor | OnSubscript | OnVar,
   ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 0c646d2c934..d5166884132 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -8711,6 +8711,19 @@ GROUPED_WARNING(
     "behavior",
     (StringRef))
 
+//===----------------------------------------------------------------------===//
+// MARK: @extensible Attribute
+//===----------------------------------------------------------------------===//
+
+ERROR(extensible_attr_on_frozen_type,none,
+      "cannot use '@extensible' together with '@frozen'", ())
+
+ERROR(extensible_attr_on_internal_type,none, 
+      "'@extensible' attribute can only be applied to public or package "
+      "declarations, but %0 is "
+      "%select{private|fileprivate|internal|%error|%error|%error}1",
+      (DeclName, AccessLevel))
+
 //===----------------------------------------------------------------------===//
 // MARK: `using` declaration
 //===----------------------------------------------------------------------===//
diff --git a/lib/AST/ASTDumper.cpp b/lib/AST/ASTDumper.cpp
index 9a9ac6fb8e1..5c7dbecfdc2 100644
--- a/lib/AST/ASTDumper.cpp
+++ b/lib/AST/ASTDumper.cpp
@@ -5064,6 +5064,7 @@ public:
   TRIVIAL_ATTR_PRINTER(Used, used)
   TRIVIAL_ATTR_PRINTER(WarnUnqualifiedAccess, warn_unqualified_access)
   TRIVIAL_ATTR_PRINTER(WeakLinked, weak_linked)
+  TRIVIAL_ATTR_PRINTER(Extensible, extensible)
   TRIVIAL_ATTR_PRINTER(Concurrent, concurrent)
 
 #undef TRIVIAL_ATTR_PRINTER
diff --git a/lib/AST/ASTPrinter.cpp b/lib/AST/ASTPrinter.cpp
index e6cf506e37d..8fcf6674872 100644
--- a/lib/AST/ASTPrinter.cpp
+++ b/lib/AST/ASTPrinter.cpp
@@ -407,6 +407,7 @@ PrintOptions PrintOptions::printSwiftInterfaceFile(ModuleDecl *ModuleToPrint,
       DeclAttrKind::RestatedObjCConformance,
       DeclAttrKind::NonSendable,
       DeclAttrKind::AllowFeatureSuppression,
+      DeclAttrKind::Extensible,
   };
 
   return result;
diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index f96299cf1c0..912d8f2d546 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -7027,14 +7027,11 @@ bool EnumDecl::treatAsExhaustiveForDiags(const DeclContext *useDC) const {
     if (enumModule->inSamePackage(useDC->getParentModule()))
       return true;
 
-    // If the module where enum is declared supports extensible enumerations
-    // and this enum is not explicitly marked as "@frozen", cross-module
-    // access cannot be exhaustive and requires `@unknown default:`.
-    if (enumModule->supportsExtensibleEnums() &&
-        !getAttrs().hasAttribute<FrozenAttr>()) {
-      if (useDC != enumModule->getDeclContext())
-        return false;
-    }
+    // When the enum is marked as `@extensible` cross-module access
+    // cannot be exhaustive and requires `@unknown default:`.
+    if (getAttrs().hasAttribute<ExtensibleAttr>() &&
+        enumModule != useDC->getParentModule())
+      return false;
   }
 
   return isFormallyExhaustive(useDC);
diff --git a/lib/ASTGen/Sources/ASTGen/DeclAttrs.swift b/lib/ASTGen/Sources/ASTGen/DeclAttrs.swift
index 268b9f74d25..10c8dc1c731 100644
--- a/lib/ASTGen/Sources/ASTGen/DeclAttrs.swift
+++ b/lib/ASTGen/Sources/ASTGen/DeclAttrs.swift
@@ -228,6 +228,7 @@ extension ASTGenVisitor {
         .dynamicCallable,
         .eagerMove,
         .exported,
+        .extensible,
         .discardableResult,
         .disfavoredOverload,
         .dynamicMemberLookup,
diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index 00b5258e7c1..9dd08f517f7 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -247,6 +247,21 @@ public:
     }
   }
 
+  void visitExtensibleAttr(ExtensibleAttr *attr) {
+    auto *E = cast<EnumDecl>(D);
+
+    if (D->getAttrs().hasAttribute<FrozenAttr>()) {
+      diagnoseAndRemoveAttr(attr, diag::extensible_attr_on_frozen_type);
+      return;
+    }
+
+    if (E->getFormalAccess() < AccessLevel::Package) {
+      diagnoseAndRemoveAttr(attr, diag::extensible_attr_on_internal_type,
+                            E->getName(), E->getFormalAccess());
+      return;
+    }
+  }
+
   void visitAlignmentAttr(AlignmentAttr *attr) {
     // Alignment must be a power of two.
     auto value = attr->getValue();
diff --git a/lib/Sema/TypeCheckDeclOverride.cpp b/lib/Sema/TypeCheckDeclOverride.cpp
index e4bce4488a7..ffe9b84c67b 100644
--- a/lib/Sema/TypeCheckDeclOverride.cpp
+++ b/lib/Sema/TypeCheckDeclOverride.cpp
@@ -1616,6 +1616,7 @@ namespace  {
     UNINTERESTING_ATTR(Isolated)
     UNINTERESTING_ATTR(Optimize)
     UNINTERESTING_ATTR(Exclusivity)
+    UNINTERESTING_ATTR(Extensible)
     UNINTERESTING_ATTR(NoLocks)
     UNINTERESTING_ATTR(NoAllocation)
     UNINTERESTING_ATTR(NoRuntime)
diff --git a/lib/Sema/TypeCheckSwitchStmt.cpp b/lib/Sema/TypeCheckSwitchStmt.cpp
index 5e5faf5d0b3..cb87f469c97 100644
--- a/lib/Sema/TypeCheckSwitchStmt.cpp
+++ b/lib/Sema/TypeCheckSwitchStmt.cpp
@@ -1159,10 +1159,7 @@ namespace {
           auto *enumModule = theEnum->getParentModule();
           shouldIncludeFutureVersionComment =
               enumModule->isSystemModule() ||
-              enumModule->supportsExtensibleEnums();
-          // Since the module enabled `ExtensibleEnums` feature they
-          // opted-in all of their clients into exhaustivity errors.
-          shouldDowngradeToWarning = !enumModule->supportsExtensibleEnums();
+              theEnum->getAttrs().hasAttribute<ExtensibleAttr>();
         }
         DE.diagnose(startLoc, diag::non_exhaustive_switch_unknown_only,
                     subjectType, shouldIncludeFutureVersionComment)
diff --git a/lib/Serialization/ModuleFormat.h b/lib/Serialization/ModuleFormat.h
index 004131ee7bd..7ec7766274c 100644
--- a/lib/Serialization/ModuleFormat.h
+++ b/lib/Serialization/ModuleFormat.h
@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 944; // nonisolated(nonsending) isolation was moved
+const uint16_t SWIFTMODULE_VERSION_MINOR = 945;  // @extensible attribute
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
diff --git a/test/IDE/complete_decl_attribute.swift b/test/IDE/complete_decl_attribute.swift
index a7104e38d3d..4c0970de2f1 100644
--- a/test/IDE/complete_decl_attribute.swift
+++ b/test/IDE/complete_decl_attribute.swift
@@ -344,6 +344,7 @@ struct _S {
 // ON_MEMBER_LAST-DAG: Keyword/None:                       freestanding[#Declaration Attribute#]; name=freestanding
 // ON_MEMBER_LAST-DAG: Keyword/None:                       storageRestrictions[#Declaration Attribute#]; name=storageRestrictions
 // ON_MEMBER_LAST-DAG: Keyword/None:                       lifetime[#Declaration Attribute#]; name=lifetime
+// ON_MEMBER_LAST-DAG: Keyword/None:                       extensible[#Declaration Attribute#]; name=extensible
 // ON_MEMBER_LAST-DAG: Keyword/None:                       concurrent[#Declaration Attribute#]; name=concurrent
 // ON_MEMBER_LAST-NOT: Keyword
 // ON_MEMBER_LAST-DAG: Decl[Struct]/CurrModule:            MyStruct[#MyStruct#]; name=MyStruct
@@ -420,6 +421,7 @@ func dummy2() {}
 // KEYWORD_LAST-DAG: Keyword/None:                       attached[#Declaration Attribute#]; name=attached
 // KEYWORD_LAST-DAG: Keyword/None:                       storageRestrictions[#Declaration Attribute#]; name=storageRestrictions
 // KEYWORD_LAST-DAG: Keyword/None:                       lifetime[#Declaration Attribute#]; name=lifetime
+// KEYWORD_LAST-DAG: Keyword/None:                       extensible[#Declaration Attribute#]; name=extensible
 // KEYWORD_LAST-DAG: Keyword/None:                       concurrent[#Declaration Attribute#]; name=concurrent
 // KEYWORD_LAST-NOT: Keyword
 // KEYWORD_LAST-DAG: Decl[Struct]/CurrModule:            MyStruct[#MyStruct#]; name=MyStruct
diff --git a/test/ModuleInterface/attrs.swift b/test/ModuleInterface/attrs.swift
index 56da19cc2e3..c7d9481c2d1 100644
--- a/test/ModuleInterface/attrs.swift
+++ b/test/ModuleInterface/attrs.swift
@@ -112,3 +112,11 @@ public struct TestPlacementOfAttrsAndSpecifiers {
   // CHECK: public func test3<T>(_: inout () async -> T)
   public func test3<T>(_: inout () async -> T) {}
 }
+
+// CHECK-NOT: @extensible
+// CHECK: public enum TestExtensible
+@extensible
+public enum TestExtensible {
+  case a
+  case b
+}
diff --git a/test/ModuleInterface/extensible_enums.swift b/test/ModuleInterface/extensible_enums.swift
index 8f3d985a32a..e23a19b6eff 100644
--- a/test/ModuleInterface/extensible_enums.swift
+++ b/test/ModuleInterface/extensible_enums.swift
@@ -5,8 +5,7 @@
 /// Build the library
 // RUN: %target-swift-frontend -emit-module %t/src/Lib.swift \
 // RUN:   -module-name Lib \
-// RUN:   -emit-module-path %t/Lib.swiftmodule \
-// RUN:   -enable-experimental-feature ExtensibleEnums
+// RUN:   -emit-module-path %t/Lib.swiftmodule
 
 // Check that the errors are produced when using enums from module with `ExtensibleEnums` feature enabled.
 // RUN: %target-swift-frontend -typecheck %t/src/TestChecking.swift \
@@ -19,8 +18,7 @@
 // RUN: %target-swift-frontend -emit-module %t/src/Lib.swift \
 // RUN:   -module-name Lib \
 // RUN:   -package-name Test \
-// RUN:   -emit-module-path %t/Lib.swiftmodule \
-// RUN:   -enable-experimental-feature ExtensibleEnums
+// RUN:   -emit-module-path %t/Lib.swiftmodule
 
 // Different module but the same package
 // RUN: %target-swift-frontend -typecheck %t/src/TestSamePackage.swift \
@@ -28,10 +26,9 @@
 // RUN:   -package-name Test \
 // RUN:   -verify
 
-// REQUIRES: swift_feature_ExtensibleEnums
-
 //--- Lib.swift
 
+@extensible
 public enum E {
   case a
 }
@@ -57,10 +54,10 @@ func test_same_module(e: E, f: F) {
 import Lib
 
 func test(e: E, f: F) {
-  // `E` is not marked as `@frozen` which means it gets new semantics
+  // `E` is marked as `@extensible` which means it gets new semantics
 
   switch e {
-  // expected-error@-1 {{switch covers known cases, but 'E' may have additional unknown values, possibly added in future versions}}
+  // expected-warning@-1 {{switch covers known cases, but 'E' may have additional unknown values, possibly added in future versions; this is an error in the Swift 6 language mode}}
   // expected-note@-2 {{handle unknown values using "@unknown default"}}
   case .a: break
   }
@@ -70,7 +67,7 @@ func test(e: E, f: F) {
   @unknown default: break
   }
 
-  // `F` is marked as `@frozen` which means regular rules apply even with `ExtensibleEnums` feature enabled.  
+  // `F` is marked as `@frozen` which means regular rules apply.
 
   switch f { // Ok (no errors because `F` is `@frozen`)
   case .a: break
diff --git a/test/attr/attr_extensible.swift b/test/attr/attr_extensible.swift
new file mode 100644
index 00000000000..60eabf9574c
--- /dev/null
+++ b/test/attr/attr_extensible.swift
@@ -0,0 +1,38 @@
+// RUN: %target-typecheck-verify-swift
+
+@extensible
+public enum E1 { // Ok
+}
+
+@extensible // expected-error {{'@extensible' attribute can only be applied to public or package declarations, but 'E2' is fileprivate}}
+fileprivate enum E2 {}
+
+@extensible // expected-error {{cannot use '@extensible' together with '@frozen'}}
+@frozen
+public enum E3 {
+}
+
+@extensible  // expected-error {{'@extensible' attribute can only be applied to public or package declarations, but 'E4' is internal}}
+@usableFromInline
+enum E4 {}
+
+@extensible // expected-error {{@extensible may only be used on 'enum' declarations}}
+struct Test {
+  @extensible // expected-error {{@extensible may only be used on 'enum' declarations}}
+  var v: Int {
+    @extensible // expected-error {{@extensible may only be used on 'enum' declarations}}
+    get { 0 }
+  }
+
+  @extensible // expected-error {{@extensible may only be used on 'enum' declarations}}
+  var v2: String = ""
+  
+  @extensible // expected-error {{@extensible may only be used on 'enum' declarations}}
+  func test() {}
+
+  @extensible // expected-error {{@extensible may only be used on 'enum' declarations}}
+  subscript(a: Int) -> Bool {
+    get { false }
+    set { }
+  }
+}