diff --git a/dio/lib/src/transformers/sync_transformer.dart b/dio/lib/src/transformers/sync_transformer.dart
index 9ddc08c83..e2c6f336c 100644
--- a/dio/lib/src/transformers/sync_transformer.dart
+++ b/dio/lib/src/transformers/sync_transformer.dart
@@ -125,11 +125,17 @@ class SyncTransformer extends Transformer {
     );
     final String? response;
     if (options.responseDecoder != null) {
-      response = await options.responseDecoder!(
+      final decodeResponse = options.responseDecoder!(
         responseBytes,
         options,
         responseBody..stream = Stream.empty(),
       );
+
+      if (decodeResponse is Future) {
+        response = await decodeResponse;
+      } else {
+        response = decodeResponse;
+      }
     } else if (!isJsonContent || responseBytes.isNotEmpty) {
       response = utf8.decode(responseBytes, allowMalformed: true);
     } else {
diff --git a/dio/test/options_test.dart b/dio/test/options_test.dart
index 0a30464c6..2c99d2200 100644
--- a/dio/test/options_test.dart
+++ b/dio/test/options_test.dart
@@ -399,6 +399,28 @@ void main() {
     expect(response.data, null);
   });
 
+  test('responseDecoder can return Future<String?>', () async {
+    final dio = Dio();
+    dio.options.responseDecoder = (_, __, ___) => Future.value('example');
+    dio.options.baseUrl = EchoAdapter.mockBase;
+    dio.httpClientAdapter = EchoAdapter();
+
+    final Response response = await dio.get('');
+
+    expect(response.data, 'example');
+  });
+
+  test('responseDecoder can return String?', () async {
+    final dio = Dio();
+    dio.options.responseDecoder = (_, __, ___) => 'example';
+    dio.options.baseUrl = EchoAdapter.mockBase;
+    dio.httpClientAdapter = EchoAdapter();
+
+    final Response response = await dio.get('');
+
+    expect(response.data, 'example');
+  });
+
   test('invalid response type throws exceptions', () async {
     final dio = Dio(
       BaseOptions(