diff --git a/packages/go_router/CHANGELOG.md b/packages/go_router/CHANGELOG.md
index ebc65a4c0cd..8819d18fb9b 100644
--- a/packages/go_router/CHANGELOG.md
+++ b/packages/go_router/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 15.3.0
+
+- Adds annotation for go_router_builder that enable custom string encoder/decoder [#110781](https://github.com/flutter/flutter/issues/110781). **Requires go_router_builder >= 3.1.0**.
+
 ## 15.2.4
 
 - Fixes routing to treat URLs with different cases (e.g., `/Home` vs `/home`) as distinct routes.
diff --git a/packages/go_router/lib/go_router.dart b/packages/go_router/lib/go_router.dart
index d2728a077c0..b16c7aa5e01 100644
--- a/packages/go_router/lib/go_router.dart
+++ b/packages/go_router/lib/go_router.dart
@@ -11,6 +11,7 @@ export 'src/configuration.dart';
 export 'src/delegate.dart';
 export 'src/information_provider.dart';
 export 'src/match.dart' hide RouteMatchListCodec;
+export 'src/misc/custom_parameter.dart';
 export 'src/misc/errors.dart';
 export 'src/misc/extensions.dart';
 export 'src/misc/inherited_router.dart';
diff --git a/packages/go_router/lib/src/misc/custom_parameter.dart b/packages/go_router/lib/src/misc/custom_parameter.dart
new file mode 100644
index 00000000000..94cd6d14888
--- /dev/null
+++ b/packages/go_router/lib/src/misc/custom_parameter.dart
@@ -0,0 +1,40 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:meta/meta_meta.dart';
+
+/// annotation to define a custom parameter decoder/encoder
+/// this is useful when the is encoded/decoded in a non-standard way like base64
+/// this must be used as an annotation on a field
+/// ```dart
+/// String fromBase64(String value) {
+///   return const Utf8Decoder().convert(base64.decode(value));
+/// }
+/// String toBase64(String value) {
+///   return base64.encode(const Utf8Encoder().convert(value));
+/// }
+/// class MyRoute {
+///   @CustomParameterCodec(
+///     encode: toBase64,
+///     decode: fromBase64,
+///   )
+///   final String data;
+///   MyRoute(this.data);
+/// }
+/// ```
+@Target(<TargetKind>{TargetKind.field})
+class CustomParameterCodec {
+  /// create a custom parameter codec
+  ///
+  const CustomParameterCodec({
+    required this.encode,
+    required this.decode,
+  });
+
+  /// custom function to encode the field
+  final String Function(String json) encode;
+
+  /// custom function to decode the field
+  final String Function(String json) decode;
+}
diff --git a/packages/go_router/pubspec.yaml b/packages/go_router/pubspec.yaml
index 6be302ef6ba..dd279a67302 100644
--- a/packages/go_router/pubspec.yaml
+++ b/packages/go_router/pubspec.yaml
@@ -1,7 +1,7 @@
 name: go_router
 description: A declarative router for Flutter based on Navigation 2 supporting
   deep linking, data-driven routes and more
-version: 15.2.4
+version: 15.3.0
 repository: https://github.com/flutter/packages/tree/main/packages/go_router
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+go_router%22
 
diff --git a/packages/go_router/test/route_data_test.dart b/packages/go_router/test/route_data_test.dart
index 218caef690d..a3b7485748d 100644
--- a/packages/go_router/test/route_data_test.dart
+++ b/packages/go_router/test/route_data_test.dart
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
 
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -226,6 +227,14 @@ final List<GoRoute> _routes = <GoRoute>[
   _goRouteDataRedirect,
 ];
 
+String fromBase64(String value) {
+  return const Utf8Decoder().convert(base64.decode(value));
+}
+
+String toBase64(String value) {
+  return base64.encode(const Utf8Encoder().convert(value));
+}
+
 void main() {
   group('GoRouteData', () {
     testWidgets(
@@ -633,4 +642,14 @@ void main() {
           ),
     );
   });
+
+  test('CustomParameterCodec with required parameters', () {
+    const CustomParameterCodec customParameterCodec = CustomParameterCodec(
+      encode: toBase64,
+      decode: fromBase64,
+    );
+
+    expect(customParameterCodec.encode, toBase64);
+    expect(customParameterCodec.decode, fromBase64);
+  });
 }