diff --git a/integration/avoid-import-conflicts/simple.ts b/integration/avoid-import-conflicts/simple.ts
index 171c0456b..a108d960e 100644
--- a/integration/avoid-import-conflicts/simple.ts
+++ b/integration/avoid-import-conflicts/simple.ts
@@ -43,8 +43,9 @@ export function simpleEnumToJSON(object: SimpleEnum): string {
       return 'LOCAL_FOO';
     case SimpleEnum.LOCAL_BAR:
       return 'LOCAL_BAR';
+    case SimpleEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/avoid-import-conflicts/simple2.ts b/integration/avoid-import-conflicts/simple2.ts
index 19efeba9a..d8d1807b2 100644
--- a/integration/avoid-import-conflicts/simple2.ts
+++ b/integration/avoid-import-conflicts/simple2.ts
@@ -37,8 +37,9 @@ export function simpleEnumToJSON(object: SimpleEnum): string {
       return 'IMPORT_FOO';
     case SimpleEnum.IMPORT_BAR:
       return 'IMPORT_BAR';
+    case SimpleEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/const-enum/const-enum.ts b/integration/const-enum/const-enum.ts
index 64cb6b6fc..d7824c589 100644
--- a/integration/const-enum/const-enum.ts
+++ b/integration/const-enum/const-enum.ts
@@ -48,8 +48,9 @@ export function dividerData_DividerTypeToJSON(object: DividerData_DividerType):
       return 'DASHED';
     case DividerData_DividerType.DOTTED:
       return 'DOTTED';
+    case DividerData_DividerType.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -63,8 +64,9 @@ export function dividerData_DividerTypeToNumber(object: DividerData_DividerType)
       return 2;
     case DividerData_DividerType.DOTTED:
       return 3;
+    case DividerData_DividerType.UNRECOGNIZED:
     default:
-      return 0;
+      return -1;
   }
 }
 
diff --git a/integration/enums-as-literals-with-string-enums/enums-as-literals-with-string-enums.ts b/integration/enums-as-literals-with-string-enums/enums-as-literals-with-string-enums.ts
index 1bcc81d48..9fea70268 100644
--- a/integration/enums-as-literals-with-string-enums/enums-as-literals-with-string-enums.ts
+++ b/integration/enums-as-literals-with-string-enums/enums-as-literals-with-string-enums.ts
@@ -49,8 +49,9 @@ export function dividerData_DividerTypeToJSON(object: DividerData_DividerType):
       return 'DASHED';
     case DividerData_DividerType.DOTTED:
       return 'DOTTED';
+    case DividerData_DividerType.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -64,8 +65,9 @@ export function dividerData_DividerTypeToNumber(object: DividerData_DividerType)
       return 2;
     case DividerData_DividerType.DOTTED:
       return 3;
+    case DividerData_DividerType.UNRECOGNIZED:
     default:
-      return 0;
+      return -1;
   }
 }
 
diff --git a/integration/enums-as-literals/enums-as-literals.ts b/integration/enums-as-literals/enums-as-literals.ts
index 00373e612..c3d27c92e 100644
--- a/integration/enums-as-literals/enums-as-literals.ts
+++ b/integration/enums-as-literals/enums-as-literals.ts
@@ -49,8 +49,9 @@ export function dividerData_DividerTypeToJSON(object: DividerData_DividerType):
       return 'DASHED';
     case DividerData_DividerType.DOTTED:
       return 'DOTTED';
+    case DividerData_DividerType.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/file-suffix/child.pb.ts b/integration/file-suffix/child.pb.ts
index 22c1b14d9..ae0e1607c 100644
--- a/integration/file-suffix/child.pb.ts
+++ b/integration/file-suffix/child.pb.ts
@@ -31,8 +31,9 @@ export function childEnumToJSON(object: ChildEnum): string {
       return 'DEFAULT';
     case ChildEnum.FOO:
       return 'FOO';
+    case ChildEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/grpc-js/google/protobuf/struct.ts b/integration/grpc-js/google/protobuf/struct.ts
index 27c84f70c..df4825442 100644
--- a/integration/grpc-js/google/protobuf/struct.ts
+++ b/integration/grpc-js/google/protobuf/struct.ts
@@ -32,8 +32,9 @@ export function nullValueToJSON(object: NullValue): string {
   switch (object) {
     case NullValue.NULL_VALUE:
       return 'NULL_VALUE';
+    case NullValue.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/grpc-web-go-server/example.ts b/integration/grpc-web-go-server/example.ts
index 32877979a..aa5eaf406 100644
--- a/integration/grpc-web-go-server/example.ts
+++ b/integration/grpc-web-go-server/example.ts
@@ -50,8 +50,9 @@ export function dashFlash_TypeToJSON(object: DashFlash_Type): string {
       return 'Warn';
     case DashFlash_Type.Error:
       return 'Error';
+    case DashFlash_Type.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/grpc-web-no-streaming-observable/example.ts b/integration/grpc-web-no-streaming-observable/example.ts
index 4ba0524e9..7f0a75273 100644
--- a/integration/grpc-web-no-streaming-observable/example.ts
+++ b/integration/grpc-web-no-streaming-observable/example.ts
@@ -52,8 +52,9 @@ export function dashFlash_TypeToJSON(object: DashFlash_Type): string {
       return 'Warn';
     case DashFlash_Type.Error:
       return 'Error';
+    case DashFlash_Type.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/grpc-web-no-streaming/example.ts b/integration/grpc-web-no-streaming/example.ts
index 52c629788..b75f246e5 100644
--- a/integration/grpc-web-no-streaming/example.ts
+++ b/integration/grpc-web-no-streaming/example.ts
@@ -50,8 +50,9 @@ export function dashFlash_TypeToJSON(object: DashFlash_Type): string {
       return 'Warn';
     case DashFlash_Type.Error:
       return 'Error';
+    case DashFlash_Type.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/grpc-web/example.ts b/integration/grpc-web/example.ts
index 33a159660..da8dd9581 100644
--- a/integration/grpc-web/example.ts
+++ b/integration/grpc-web/example.ts
@@ -52,8 +52,9 @@ export function dashFlash_TypeToJSON(object: DashFlash_Type): string {
       return 'Warn';
     case DashFlash_Type.Error:
       return 'Error';
+    case DashFlash_Type.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/nice-grpc/google/protobuf/struct.ts b/integration/nice-grpc/google/protobuf/struct.ts
index 3e1c597ad..29a16fca1 100644
--- a/integration/nice-grpc/google/protobuf/struct.ts
+++ b/integration/nice-grpc/google/protobuf/struct.ts
@@ -32,8 +32,9 @@ export function nullValueToJSON(object: NullValue): string {
   switch (object) {
     case NullValue.NULL_VALUE:
       return 'NULL_VALUE';
+    case NullValue.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/oneof-properties/oneof.ts b/integration/oneof-properties/oneof.ts
index d937ae5e5..4bf89b448 100644
--- a/integration/oneof-properties/oneof.ts
+++ b/integration/oneof-properties/oneof.ts
@@ -63,8 +63,9 @@ export function pleaseChoose_StateEnumToJSON(object: PleaseChoose_StateEnum): st
       return 'ON';
     case PleaseChoose_StateEnum.OFF:
       return 'OFF';
+    case PleaseChoose_StateEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/oneof-unions/google/protobuf/struct.ts b/integration/oneof-unions/google/protobuf/struct.ts
index 33212f88a..5bdacf388 100644
--- a/integration/oneof-unions/google/protobuf/struct.ts
+++ b/integration/oneof-unions/google/protobuf/struct.ts
@@ -32,8 +32,9 @@ export function nullValueToJSON(object: NullValue): string {
   switch (object) {
     case NullValue.NULL_VALUE:
       return 'NULL_VALUE';
+    case NullValue.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/oneof-unions/oneof.ts b/integration/oneof-unions/oneof.ts
index c76d68a4d..186457422 100644
--- a/integration/oneof-unions/oneof.ts
+++ b/integration/oneof-unions/oneof.ts
@@ -56,8 +56,9 @@ export function pleaseChoose_StateEnumToJSON(object: PleaseChoose_StateEnum): st
       return 'ON';
     case PleaseChoose_StateEnum.OFF:
       return 'OFF';
+    case PleaseChoose_StateEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/simple-optionals/simple.ts b/integration/simple-optionals/simple.ts
index 1f18885b8..cd82b257d 100644
--- a/integration/simple-optionals/simple.ts
+++ b/integration/simple-optionals/simple.ts
@@ -45,8 +45,9 @@ export function stateEnumToJSON(object: StateEnum): string {
       return 'ON';
     case StateEnum.OFF:
       return 'OFF';
+    case StateEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -106,8 +107,9 @@ export function child_TypeToJSON(object: Child_Type): string {
       return 'GOOD';
     case Child_Type.BAD:
       return 'BAD';
+    case Child_Type.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -150,8 +152,9 @@ export function nested_InnerEnumToJSON(object: Nested_InnerEnum): string {
       return 'GOOD';
     case Nested_InnerEnum.BAD:
       return 'BAD';
+    case Nested_InnerEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/simple-proto2/simple.ts b/integration/simple-proto2/simple.ts
index 0fa06feb3..d8b9a60e0 100644
--- a/integration/simple-proto2/simple.ts
+++ b/integration/simple-proto2/simple.ts
@@ -31,8 +31,9 @@ export function enumWithoutZeroToJSON(object: EnumWithoutZero): string {
       return 'A';
     case EnumWithoutZero.B:
       return 'B';
+    case EnumWithoutZero.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/simple-prototype-defaults/simple.ts b/integration/simple-prototype-defaults/simple.ts
index d1f431dfb..c62c070b7 100644
--- a/integration/simple-prototype-defaults/simple.ts
+++ b/integration/simple-prototype-defaults/simple.ts
@@ -46,8 +46,9 @@ export function stateEnumToJSON(object: StateEnum): string {
       return 'ON';
     case StateEnum.OFF:
       return 'OFF';
+    case StateEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -110,8 +111,9 @@ export function child_TypeToJSON(object: Child_Type): string {
       return 'GOOD';
     case Child_Type.BAD:
       return 'BAD';
+    case Child_Type.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -154,8 +156,9 @@ export function nested_InnerEnumToJSON(object: Nested_InnerEnum): string {
       return 'GOOD';
     case Nested_InnerEnum.BAD:
       return 'BAD';
+    case Nested_InnerEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/simple-snake/simple.ts b/integration/simple-snake/simple.ts
index 28d415e40..ed04acc4f 100644
--- a/integration/simple-snake/simple.ts
+++ b/integration/simple-snake/simple.ts
@@ -45,8 +45,9 @@ export function stateEnumToJSON(object: StateEnum): string {
       return 'ON';
     case StateEnum.OFF:
       return 'OFF';
+    case StateEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -106,8 +107,9 @@ export function child_TypeToJSON(object: Child_Type): string {
       return 'GOOD';
     case Child_Type.BAD:
       return 'BAD';
+    case Child_Type.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -150,8 +152,9 @@ export function nested_InnerEnumToJSON(object: Nested_InnerEnum): string {
       return 'GOOD';
     case Nested_InnerEnum.BAD:
       return 'BAD';
+    case Nested_InnerEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/simple-string-enums/google/protobuf/struct.ts b/integration/simple-string-enums/google/protobuf/struct.ts
index 90af94eab..98852c543 100644
--- a/integration/simple-string-enums/google/protobuf/struct.ts
+++ b/integration/simple-string-enums/google/protobuf/struct.ts
@@ -32,8 +32,9 @@ export function nullValueToJSON(object: NullValue): string {
   switch (object) {
     case NullValue.NULL_VALUE:
       return 'NULL_VALUE';
+    case NullValue.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -41,8 +42,9 @@ export function nullValueToNumber(object: NullValue): number {
   switch (object) {
     case NullValue.NULL_VALUE:
       return 0;
+    case NullValue.UNRECOGNIZED:
     default:
-      return 0;
+      return -1;
   }
 }
 
diff --git a/integration/simple-string-enums/simple.ts b/integration/simple-string-enums/simple.ts
index b1c9f0e2c..42a34bf21 100644
--- a/integration/simple-string-enums/simple.ts
+++ b/integration/simple-string-enums/simple.ts
@@ -38,8 +38,9 @@ export function stateEnumToJSON(object: StateEnum): string {
       return 'ON';
     case StateEnum.OFF:
       return 'OFF';
+    case StateEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -51,8 +52,9 @@ export function stateEnumToNumber(object: StateEnum): number {
       return 2;
     case StateEnum.OFF:
       return 3;
+    case StateEnum.UNRECOGNIZED:
     default:
-      return 0;
+      return -1;
   }
 }
 
diff --git a/integration/simple-unrecognized-enum/simple.ts b/integration/simple-unrecognized-enum/simple.ts
index 303adbb3a..3dc4afc12 100644
--- a/integration/simple-unrecognized-enum/simple.ts
+++ b/integration/simple-unrecognized-enum/simple.ts
@@ -43,7 +43,7 @@ export function stateEnumToJSON(object: StateEnum): string {
     case StateEnum.OFF:
       return 'OFF';
     default:
-      return 'UNKNOWN';
+      throw new globalThis.Error('Unrecognized enum value ' + object + ' for enum StateEnum');
   }
 }
 
@@ -101,7 +101,7 @@ export function child_TypeToJSON(object: Child_Type): string {
     case Child_Type.BAD:
       return 'BAD';
     default:
-      return 'UNKNOWN';
+      throw new globalThis.Error('Unrecognized enum value ' + object + ' for enum Child_Type');
   }
 }
 
@@ -142,7 +142,7 @@ export function nested_InnerEnumToJSON(object: Nested_InnerEnum): string {
     case Nested_InnerEnum.BAD:
       return 'BAD';
     default:
-      return 'UNKNOWN';
+      throw new globalThis.Error('Unrecognized enum value ' + object + ' for enum Nested_InnerEnum');
   }
 }
 
diff --git a/integration/simple/simple.ts b/integration/simple/simple.ts
index 278506635..cf4b12fb7 100644
--- a/integration/simple/simple.ts
+++ b/integration/simple/simple.ts
@@ -46,8 +46,9 @@ export function stateEnumToJSON(object: StateEnum): string {
       return 'ON';
     case StateEnum.OFF:
       return 'OFF';
+    case StateEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -110,8 +111,9 @@ export function child_TypeToJSON(object: Child_Type): string {
       return 'GOOD';
     case Child_Type.BAD:
       return 'BAD';
+    case Child_Type.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -154,8 +156,9 @@ export function nested_InnerEnumToJSON(object: Nested_InnerEnum): string {
       return 'GOOD';
     case Nested_InnerEnum.BAD:
       return 'BAD';
+    case Nested_InnerEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/struct/google/protobuf/struct.ts b/integration/struct/google/protobuf/struct.ts
index 27c84f70c..df4825442 100644
--- a/integration/struct/google/protobuf/struct.ts
+++ b/integration/struct/google/protobuf/struct.ts
@@ -32,8 +32,9 @@ export function nullValueToJSON(object: NullValue): string {
   switch (object) {
     case NullValue.NULL_VALUE:
       return 'NULL_VALUE';
+    case NullValue.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/type-registry/google/protobuf/struct.ts b/integration/type-registry/google/protobuf/struct.ts
index 361aa8936..1ae57045b 100644
--- a/integration/type-registry/google/protobuf/struct.ts
+++ b/integration/type-registry/google/protobuf/struct.ts
@@ -33,8 +33,9 @@ export function nullValueToJSON(object: NullValue): string {
   switch (object) {
     case NullValue.NULL_VALUE:
       return 'NULL_VALUE';
+    case NullValue.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/use-optionals-all/test.ts b/integration/use-optionals-all/test.ts
index 396e4ccd7..805535799 100644
--- a/integration/use-optionals-all/test.ts
+++ b/integration/use-optionals-all/test.ts
@@ -37,8 +37,9 @@ export function stateEnumToJSON(object: StateEnum): string {
       return 'ON';
     case StateEnum.OFF:
       return 'OFF';
+    case StateEnum.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/value/google/protobuf/struct.ts b/integration/value/google/protobuf/struct.ts
index 27c84f70c..df4825442 100644
--- a/integration/value/google/protobuf/struct.ts
+++ b/integration/value/google/protobuf/struct.ts
@@ -32,8 +32,9 @@ export function nullValueToJSON(object: NullValue): string {
   switch (object) {
     case NullValue.NULL_VALUE:
       return 'NULL_VALUE';
+    case NullValue.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/integration/vector-tile/vector_tile.ts b/integration/vector-tile/vector_tile.ts
index 122834fc1..ef61a98a5 100644
--- a/integration/vector-tile/vector_tile.ts
+++ b/integration/vector-tile/vector_tile.ts
@@ -47,8 +47,9 @@ export function tile_GeomTypeToJSON(object: Tile_GeomType): string {
       return 'LINESTRING';
     case Tile_GeomType.POLYGON:
       return 'POLYGON';
+    case Tile_GeomType.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/protos/google/protobuf/compiler/plugin.ts b/protos/google/protobuf/compiler/plugin.ts
index 756f60b02..91688ecfa 100644
--- a/protos/google/protobuf/compiler/plugin.ts
+++ b/protos/google/protobuf/compiler/plugin.ts
@@ -95,8 +95,9 @@ export function codeGeneratorResponse_FeatureToJSON(object: CodeGeneratorRespons
       return 'FEATURE_NONE';
     case CodeGeneratorResponse_Feature.FEATURE_PROTO3_OPTIONAL:
       return 'FEATURE_PROTO3_OPTIONAL';
+    case CodeGeneratorResponse_Feature.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/protos/google/protobuf/descriptor.ts b/protos/google/protobuf/descriptor.ts
index 31a11b186..852014707 100644
--- a/protos/google/protobuf/descriptor.ts
+++ b/protos/google/protobuf/descriptor.ts
@@ -303,8 +303,9 @@ export function fieldDescriptorProto_TypeToJSON(object: FieldDescriptorProto_Typ
       return 'TYPE_SINT32';
     case FieldDescriptorProto_Type.TYPE_SINT64:
       return 'TYPE_SINT64';
+    case FieldDescriptorProto_Type.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -342,8 +343,9 @@ export function fieldDescriptorProto_LabelToJSON(object: FieldDescriptorProto_La
       return 'LABEL_REQUIRED';
     case FieldDescriptorProto_Label.LABEL_REPEATED:
       return 'LABEL_REPEATED';
+    case FieldDescriptorProto_Label.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -574,8 +576,9 @@ export function fileOptions_OptimizeModeToJSON(object: FileOptions_OptimizeMode)
       return 'CODE_SIZE';
     case FileOptions_OptimizeMode.LITE_RUNTIME:
       return 'LITE_RUNTIME';
+    case FileOptions_OptimizeMode.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -750,8 +753,9 @@ export function fieldOptions_CTypeToJSON(object: FieldOptions_CType): string {
       return 'CORD';
     case FieldOptions_CType.STRING_PIECE:
       return 'STRING_PIECE';
+    case FieldOptions_CType.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -791,8 +795,9 @@ export function fieldOptions_JSTypeToJSON(object: FieldOptions_JSType): string {
       return 'JS_STRING';
     case FieldOptions_JSType.JS_NUMBER:
       return 'JS_NUMBER';
+    case FieldOptions_JSType.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
@@ -895,8 +900,9 @@ export function methodOptions_IdempotencyLevelToJSON(object: MethodOptions_Idemp
       return 'NO_SIDE_EFFECTS';
     case MethodOptions_IdempotencyLevel.IDEMPOTENT:
       return 'IDEMPOTENT';
+    case MethodOptions_IdempotencyLevel.UNRECOGNIZED:
     default:
-      return 'UNKNOWN';
+      return 'UNRECOGNIZED';
   }
 }
 
diff --git a/src/enums.ts b/src/enums.ts
index 7cc079dbb..a1ad7620a 100644
--- a/src/enums.ts
+++ b/src/enums.ts
@@ -56,11 +56,11 @@ export function generateEnum(
   }
   if (options.outputJsonMethods) {
     chunks.push(code`\n`);
-    chunks.push(generateEnumToJson(fullName, enumDesc));
+    chunks.push(generateEnumToJson(ctx, fullName, enumDesc));
   }
   if (options.stringEnums && options.outputEncodeMethods) {
     chunks.push(code`\n`);
-    chunks.push(generateEnumToNumber(fullName, enumDesc));
+    chunks.push(generateEnumToNumber(ctx, fullName, enumDesc));
   }
 
   return joinCode(chunks, { on: '\n' });
@@ -104,7 +104,9 @@ export function generateEnumFromJson(ctx: Context, fullName: string, enumDesc: E
 }
 
 /** Generates a function with a big switch statement to encode our enum -> JSON. */
-export function generateEnumToJson(fullName: string, enumDesc: EnumDescriptorProto): Code {
+export function generateEnumToJson(ctx: Context, fullName: string, enumDesc: EnumDescriptorProto): Code {
+  const { options, utils } = ctx;
+
   const chunks: Code[] = [];
 
   const functionName = camelCase(fullName) + 'ToJSON';
@@ -114,7 +116,20 @@ export function generateEnumToJson(fullName: string, enumDesc: EnumDescriptorPro
   for (const valueDesc of enumDesc.value) {
     chunks.push(code`case ${fullName}.${valueDesc.name}: return "${valueDesc.name}";`);
   }
-  chunks.push(code`default: return "UNKNOWN";`);
+
+  if (options.unrecognizedEnum) {
+    chunks.push(code`
+      case ${fullName}.${UNRECOGNIZED_ENUM_NAME}:
+      default:
+        return "${UNRECOGNIZED_ENUM_NAME}";
+    `);
+  } else {
+    // We use globalThis to avoid conflicts on protobuf types named `Error`.
+    chunks.push(code`
+      default:
+        throw new ${utils.globalThis}.Error("Unrecognized enum value " + object + " for enum ${fullName}");
+    `);
+  }
 
   chunks.push(code`}`);
   chunks.push(code`}`);
@@ -122,7 +137,9 @@ export function generateEnumToJson(fullName: string, enumDesc: EnumDescriptorPro
 }
 
 /** Generates a function with a big switch statement to encode our string enum -> int value. */
-export function generateEnumToNumber(fullName: string, enumDesc: EnumDescriptorProto): Code {
+export function generateEnumToNumber(ctx: Context, fullName: string, enumDesc: EnumDescriptorProto): Code {
+  const { options, utils } = ctx;
+
   const chunks: Code[] = [];
 
   const functionName = camelCase(fullName) + 'ToNumber';
@@ -131,7 +148,21 @@ export function generateEnumToNumber(fullName: string, enumDesc: EnumDescriptorP
   for (const valueDesc of enumDesc.value) {
     chunks.push(code`case ${fullName}.${valueDesc.name}: return ${valueDesc.number};`);
   }
-  chunks.push(code`default: return 0;`);
+
+  if (options.unrecognizedEnum) {
+    chunks.push(code`
+      case ${fullName}.${UNRECOGNIZED_ENUM_NAME}:
+      default:
+        return ${UNRECOGNIZED_ENUM_VALUE};
+    `);
+  } else {
+    // We use globalThis to avoid conflicts on protobuf types named `Error`.
+    chunks.push(code`
+      default:
+        throw new ${utils.globalThis}.Error("Unrecognized enum value " + object + " for enum ${fullName}");
+    `);
+  }
+
   chunks.push(code`}`);
   chunks.push(code`}`);
   return joinCode(chunks, { on: '\n' });