diff --git a/dio/CHANGELOG.md b/dio/CHANGELOG.md
index 306a2b79f..786414884 100644
--- a/dio/CHANGELOG.md
+++ b/dio/CHANGELOG.md
@@ -6,6 +6,7 @@ See the [Migration Guide][] for the complete breaking changes list.**
 ## Unreleased
 
 - Improve `SyncTransformer`'s stream transform.
+- Allow case-sensitive header keys with the `preserveHeaderCase` flag through options.
 
 ## 5.3.4
 
diff --git a/dio/README-ZH.md b/dio/README-ZH.md
index 6ccca968a..a3819f000 100644
--- a/dio/README-ZH.md
+++ b/dio/README-ZH.md
@@ -291,8 +291,20 @@ String method;
 String? baseUrl;
 
 /// HTTP 请求头。
+///
+/// 请求头的键是否相等的判断大小写不敏感的。
+/// 例如：`content-type` 和 `Content-Type` 会视为同样的请求头键。
 Map<String, dynamic>? headers;
 
+/// 是否保留请求头的大小写。
+///
+/// 默认值为 false。
+///
+/// 该选项在以下场景无效：
+///  - XHR 不支持直接处理。
+///  - 按照 HTTP/2 的标准，只支持小写请求头键。
+bool? preserveHeaderCase;
+
 /// 连接服务器超时时间.
 Duration? connectTimeout;
 
diff --git a/dio/README.md b/dio/README.md
index c415d8f72..4a268d1c2 100644
--- a/dio/README.md
+++ b/dio/README.md
@@ -275,8 +275,20 @@ String method;
 String? baseUrl;
 
 /// Http request headers.
+///
+/// The equality of the header keys is case-insensitive,
+/// e.g.: `content-type` and `Content-Type` will be treated as the same key.
 Map<String, dynamic>? headers;
 
+/// Whether the case of header keys should be preserved.
+///
+/// Defaults to false.
+///
+/// This option WILL NOT take effect on these circumstances:
+/// - XHR ([HttpRequest]) does not support handling this explicitly.
+/// - The HTTP/2 standard only supports lowercase header keys.
+bool? preserveHeaderCase;
+
 /// Timeout for opening url.
 Duration? connectTimeout;
 
diff --git a/dio/lib/src/adapters/io_adapter.dart b/dio/lib/src/adapters/io_adapter.dart
index 35a1b781b..5c7d94ece 100644
--- a/dio/lib/src/adapters/io_adapter.dart
+++ b/dio/lib/src/adapters/io_adapter.dart
@@ -111,8 +111,14 @@ class IOHttpClientAdapter implements HttpClientAdapter {
       }
 
       // Set Headers
-      options.headers.forEach((k, v) {
-        if (v != null) request.headers.set(k, v);
+      options.headers.forEach((key, value) {
+        if (value != null) {
+          request.headers.set(
+            key,
+            value,
+            preserveHeaderCase: options.preserveHeaderCase,
+          );
+        }
       });
     } on SocketException catch (e) {
       if (e.message.contains('timed out')) {
diff --git a/dio/lib/src/dio_mixin.dart b/dio/lib/src/dio_mixin.dart
index 0387540a4..66973d5ba 100644
--- a/dio/lib/src/dio_mixin.dart
+++ b/dio/lib/src/dio_mixin.dart
@@ -517,7 +517,10 @@ abstract class DioMixin implements Dio {
         stream,
         cancelToken?.whenCancel,
       );
-      final headers = Headers.fromMap(responseBody.headers);
+      final headers = Headers.fromMap(
+        responseBody.headers,
+        preserveHeaderCase: reqOpt.preserveHeaderCase,
+      );
       // Make sure headers and [ResponseBody.headers] are the same instance.
       responseBody.headers = headers.map;
       final ret = Response<dynamic>(
@@ -713,7 +716,10 @@ abstract class DioMixin implements Dio {
       final T? data = response.data as T?;
       final Headers headers;
       if (data is ResponseBody) {
-        headers = Headers.fromMap(data.headers);
+        headers = Headers.fromMap(
+          data.headers,
+          preserveHeaderCase: requestOptions.preserveHeaderCase,
+        );
       } else {
         headers = response.headers;
       }
diff --git a/dio/lib/src/headers.dart b/dio/lib/src/headers.dart
index 42854c3e2..d0d9524a8 100644
--- a/dio/lib/src/headers.dart
+++ b/dio/lib/src/headers.dart
@@ -7,12 +7,16 @@ typedef HeaderForEachCallback = void Function(String name, List<String> values);
 
 /// The headers class for requests and responses.
 class Headers {
-  Headers() : _map = caseInsensitiveKeyMap<List<String>>();
+  Headers({
+    this.preserveHeaderCase = false,
+  }) : _map = caseInsensitiveKeyMap<List<String>>();
 
   /// Create the [Headers] from a [Map] instance.
-  Headers.fromMap(Map<String, List<String>> map)
-      : _map = caseInsensitiveKeyMap<List<String>>(
-          map.map((k, v) => MapEntry(k.trim().toLowerCase(), v)),
+  Headers.fromMap(
+    Map<String, List<String>> map, {
+    this.preserveHeaderCase = false,
+  }) : _map = caseInsensitiveKeyMap<List<String>>(
+          map.map((k, v) => MapEntry(k.trim(), v)),
         );
 
   static const acceptHeader = 'accept';
@@ -28,6 +32,11 @@ class Headers {
 
   static final jsonMimeType = MediaType.parse(jsonContentType);
 
+  /// Whether the header key should be case-sensitive.
+  ///
+  /// Defaults to false.
+  final bool preserveHeaderCase;
+
   final Map<String, List<String>> _map;
 
   Map<String, List<String>> get map => _map;
@@ -35,7 +44,7 @@ class Headers {
   /// Returns the list of values for the header named [name]. If there
   /// is no header with the provided name, [:null:] will be returned.
   List<String>? operator [](String name) {
-    return _map[name.trim().toLowerCase()];
+    return _map[name.trim()];
   }
 
   /// Convenience method for the value for a single valued header. If
@@ -63,9 +72,9 @@ class Headers {
   /// cleared before the value [value] is added as its value.
   void set(String name, dynamic value) {
     if (value == null) return;
-    name = name.trim().toLowerCase();
+    name = name.trim();
     if (value is List) {
-      _map[name] = value.map<String>((e) => e.toString()).toList();
+      _map[name] = value.map<String>((e) => '$e').toList();
     } else {
       _map[name] = ['$value'.trim()];
     }
diff --git a/dio/lib/src/options.dart b/dio/lib/src/options.dart
index 44038d380..b95994f7c 100644
--- a/dio/lib/src/options.dart
+++ b/dio/lib/src/options.dart
@@ -130,6 +130,7 @@ class BaseOptions extends _RequestConfig with OptionsMixin {
     Map<String, dynamic>? queryParameters,
     Map<String, dynamic>? extra,
     Map<String, dynamic>? headers,
+    bool preserveHeaderCase = false,
     ResponseType? responseType = ResponseType.json,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -148,6 +149,7 @@ class BaseOptions extends _RequestConfig with OptionsMixin {
           sendTimeout: sendTimeout,
           extra: extra,
           headers: headers,
+          preserveHeaderCase: preserveHeaderCase,
           responseType: responseType,
           contentType: contentType,
           validateStatus: validateStatus,
@@ -175,6 +177,7 @@ class BaseOptions extends _RequestConfig with OptionsMixin {
     Duration? sendTimeout,
     Map<String, Object?>? extra,
     Map<String, Object?>? headers,
+    bool? preserveHeaderCase,
     ResponseType? responseType,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -195,6 +198,7 @@ class BaseOptions extends _RequestConfig with OptionsMixin {
       sendTimeout: sendTimeout ?? this.sendTimeout,
       extra: extra ?? Map.from(this.extra),
       headers: headers ?? Map.from(this.headers),
+      preserveHeaderCase: preserveHeaderCase ?? this.preserveHeaderCase,
       responseType: responseType ?? this.responseType,
       contentType: contentType ?? this.contentType,
       validateStatus: validateStatus ?? this.validateStatus,
@@ -218,6 +222,7 @@ class Options {
     Duration? receiveTimeout,
     this.extra,
     this.headers,
+    this.preserveHeaderCase,
     this.responseType,
     this.contentType,
     this.validateStatus,
@@ -240,6 +245,7 @@ class Options {
     Duration? receiveTimeout,
     Map<String, Object?>? extra,
     Map<String, Object?>? headers,
+    bool? preserveHeaderCase,
     ResponseType? responseType,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -276,6 +282,7 @@ class Options {
       receiveTimeout: receiveTimeout ?? this.receiveTimeout,
       extra: extra ?? effectiveExtra,
       headers: headers ?? effectiveHeaders,
+      preserveHeaderCase: preserveHeaderCase ?? this.preserveHeaderCase,
       responseType: responseType ?? this.responseType,
       contentType: contentType ?? this.contentType,
       validateStatus: validateStatus ?? this.validateStatus,
@@ -324,6 +331,7 @@ class Options {
       baseUrl: baseOpt.baseUrl,
       path: path,
       data: data,
+      preserveHeaderCase: preserveHeaderCase ?? baseOpt.preserveHeaderCase,
       sourceStackTrace: sourceStackTrace ?? StackTrace.current,
       connectTimeout: baseOpt.connectTimeout,
       sendTimeout: sendTimeout ?? baseOpt.sendTimeout,
@@ -354,10 +362,19 @@ class Options {
 
   /// HTTP request headers.
   ///
-  /// The keys of the header are case-insensitive,
-  /// e.g.: content-type and Content-Type will be treated as the same key.
+  /// The equality of the header keys is case-insensitive,
+  /// e.g.: `content-type` and `Content-Type` will be treated as the same key.
   Map<String, dynamic>? headers;
 
+  /// Whether the case of header keys should be preserved.
+  ///
+  /// Defaults to false.
+  ///
+  /// This option WILL NOT take effect on these circumstances:
+  /// - XHR ([HttpRequest]) does not support handling this explicitly.
+  /// - The HTTP/2 standard only supports lowercase header keys.
+  bool? preserveHeaderCase;
+
   /// Timeout when sending data.
   ///
   /// [Dio] will throw the [DioException] with
@@ -474,6 +491,7 @@ class RequestOptions extends _RequestConfig with OptionsMixin {
     String? baseUrl,
     Map<String, dynamic>? extra,
     Map<String, dynamic>? headers,
+    bool? preserveHeaderCase,
     ResponseType? responseType,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -493,6 +511,7 @@ class RequestOptions extends _RequestConfig with OptionsMixin {
           receiveTimeout: receiveTimeout,
           extra: extra,
           headers: headers,
+          preserveHeaderCase: preserveHeaderCase,
           responseType: responseType,
           contentType: contentType,
           validateStatus: validateStatus,
@@ -525,6 +544,7 @@ class RequestOptions extends _RequestConfig with OptionsMixin {
     CancelToken? cancelToken,
     Map<String, dynamic>? extra,
     Map<String, dynamic>? headers,
+    bool? preserveHeaderCase,
     ResponseType? responseType,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -561,6 +581,7 @@ class RequestOptions extends _RequestConfig with OptionsMixin {
       cancelToken: cancelToken ?? this.cancelToken,
       extra: extra ?? Map.from(this.extra),
       headers: headers ?? Map.from(this.headers),
+      preserveHeaderCase: preserveHeaderCase ?? this.preserveHeaderCase,
       responseType: responseType ?? this.responseType,
       validateStatus: validateStatus ?? this.validateStatus,
       receiveDataWhenStatusError:
@@ -638,6 +659,7 @@ class _RequestConfig {
     String? method,
     Map<String, dynamic>? extra,
     Map<String, dynamic>? headers,
+    bool? preserveHeaderCase,
     String? contentType,
     ListFormat? listFormat,
     bool? followRedirects,
@@ -653,6 +675,7 @@ class _RequestConfig {
         assert(sendTimeout == null || !sendTimeout.isNegative),
         _sendTimeout = sendTimeout,
         method = method ?? 'GET',
+        preserveHeaderCase = preserveHeaderCase ?? false,
         listFormat = listFormat ?? ListFormat.multi,
         extra = extra ?? {},
         followRedirects = followRedirects ?? true,
@@ -692,6 +715,8 @@ class _RequestConfig {
     }
   }
 
+  late bool preserveHeaderCase;
+
   Duration? get sendTimeout => _sendTimeout;
   Duration? _sendTimeout;
 
diff --git a/dio/lib/src/response.dart b/dio/lib/src/response.dart
index b6ff998de..4713330f2 100644
--- a/dio/lib/src/response.dart
+++ b/dio/lib/src/response.dart
@@ -19,7 +19,8 @@ class Response<T> {
     this.redirects = const [],
     Map<String, dynamic>? extra,
     Headers? headers,
-  })  : headers = headers ?? Headers(),
+  })  : headers = headers ??
+            Headers(preserveHeaderCase: requestOptions.preserveHeaderCase),
         extra = extra ?? <String, dynamic>{};
 
   /// The response payload in specific type.
diff --git a/dio/test/basic_test.dart b/dio/test/basic_test.dart
index e52faeae6..272d4f997 100644
--- a/dio/test/basic_test.dart
+++ b/dio/test/basic_test.dart
@@ -8,36 +8,6 @@ import 'mock/adapters.dart';
 import 'utils.dart';
 
 void main() {
-  test('test headers', () {
-    final headers = Headers.fromMap({
-      'set-cookie': ['k=v', 'k1=v1'],
-      'content-length': ['200'],
-      'test': ['1', '2'],
-    });
-    headers.add('SET-COOKIE', 'k2=v2');
-    expect(headers.value('content-length'), '200');
-    expect(Future(() => headers.value('test')), throwsException);
-    expect(headers['set-cookie']?.length, 3);
-    headers.remove('set-cookie', 'k=v');
-    expect(headers['set-cookie']?.length, 2);
-    headers.removeAll('set-cookie');
-    expect(headers['set-cookie'], isNull);
-    final ls = [];
-    headers.forEach((k, list) => ls.addAll(list));
-    expect(ls.length, 3);
-    expect(headers.toString(), 'content-length: 200\ntest: 1\ntest: 2\n');
-    headers.set('content-length', '300');
-    expect(headers.value('content-length'), '300');
-    headers.set('content-length', ['400']);
-    expect(headers.value('content-length'), '400');
-
-    final headers1 = Headers();
-    headers1.set('xx', 'v');
-    expect(headers1.value('xx'), 'v');
-    headers1.clear();
-    expect(headers1.map.isEmpty, isTrue);
-  });
-
   test('send with an invalid URL', () async {
     await expectLater(
       Dio().get('http://http.invalid'),
diff --git a/dio/test/headers_test.dart b/dio/test/headers_test.dart
new file mode 100644
index 000000000..4d71553fe
--- /dev/null
+++ b/dio/test/headers_test.dart
@@ -0,0 +1,56 @@
+import 'dart:async';
+
+import 'package:dio/dio.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group(Headers, () {
+    test('set', () {
+      final headers = Headers.fromMap({
+        'set-cookie': ['k=v', 'k1=v1'],
+        'content-length': ['200'],
+        'test': ['1', '2'],
+      });
+      headers.add('SET-COOKIE', 'k2=v2');
+      expect(headers.value('content-length'), '200');
+      expect(Future(() => headers.value('test')), throwsException);
+      expect(headers['set-cookie']?.length, 3);
+      headers.remove('set-cookie', 'k=v');
+      expect(headers['set-cookie']?.length, 2);
+      headers.removeAll('set-cookie');
+      expect(headers['set-cookie'], isNull);
+      final ls = [];
+      headers.forEach((k, list) => ls.addAll(list));
+      expect(ls.length, 3);
+      expect(headers.toString(), 'content-length: 200\ntest: 1\ntest: 2\n');
+      headers.set('content-length', '300');
+      expect(headers.value('content-length'), '300');
+      headers.set('content-length', ['400']);
+      expect(headers.value('content-length'), '400');
+    });
+
+    test('clear', () {
+      final headers1 = Headers();
+      headers1.set('xx', 'v');
+      expect(headers1.value('xx'), 'v');
+      headers1.clear();
+      expect(headers1.map.isEmpty, isTrue);
+    });
+
+    test('case-sensitive', () {
+      final headers = Headers.fromMap(
+        {
+          'SET-COOKIE': ['k=v', 'k1=v1'],
+          'content-length': ['200'],
+          'Test': ['1', '2'],
+        },
+        preserveHeaderCase: true,
+      );
+      expect(headers['SET-COOKIE']?.length, 2);
+      // Although it's case-sensitive, we still use case-insensitive map.
+      expect(headers['set-cookie']?.length, 2);
+      expect(headers['content-length']?.length, 1);
+      expect(headers['Test']?.length, 2);
+    });
+  });
+}
diff --git a/dio/test/options_test.dart b/dio/test/options_test.dart
index c31229bf9..086fe8b34 100644
--- a/dio/test/options_test.dart
+++ b/dio/test/options_test.dart
@@ -1,10 +1,15 @@
 @TestOn('vm')
 import 'dart:convert';
+import 'dart:typed_data';
 
 import 'package:dio/dio.dart';
+import 'package:dio/io.dart';
+import 'package:dio/src/utils.dart';
+import 'package:mockito/mockito.dart';
 import 'package:test/test.dart';
 
 import 'mock/adapters.dart';
+import 'mock/http_mock.mocks.dart';
 import 'utils.dart';
 
 void main() {
@@ -523,4 +528,62 @@ void main() {
       expect(response.data, 'test');
     }
   });
+
+  test('Headers can be case-sensitive', () async {
+    final dio = Dio();
+    final client = MockHttpClient();
+    dio.httpClientAdapter = IOHttpClientAdapter(createHttpClient: () => client);
+
+    final headerMap = caseInsensitiveKeyMap();
+    late MockHttpHeaders mockRequestHeaders;
+    when(client.openUrl(any, any)).thenAnswer((_) async {
+      final request = MockHttpClientRequest();
+      final response = MockHttpClientResponse();
+      mockRequestHeaders = MockHttpHeaders();
+      when(
+        mockRequestHeaders.set(
+          any,
+          any,
+          preserveHeaderCase: anyNamed('preserveHeaderCase'),
+        ),
+      ).thenAnswer((invocation) {
+        final args = invocation.positionalArguments.cast<String>();
+        final preserveHeaderCase =
+            invocation.namedArguments[#preserveHeaderCase] as bool;
+        headerMap[preserveHeaderCase ? args[0] : args[0].toLowerCase()] =
+            args[1];
+      });
+      when(request.headers).thenAnswer((_) => mockRequestHeaders);
+      when(request.close()).thenAnswer((_) => Future.value(response));
+      when(request.addStream(any)).thenAnswer((_) async => null);
+      when(response.headers).thenReturn(MockHttpHeaders());
+      when(response.statusCode).thenReturn(200);
+      when(response.reasonPhrase).thenReturn('OK');
+      when(response.isRedirect).thenReturn(false);
+      when(response.redirects).thenReturn([]);
+      when(response.transform(any))
+          .thenAnswer((_) => Stream<Uint8List>.empty());
+      return Future.value(request);
+    });
+
+    await dio.get(
+      '',
+      options: Options(
+        preserveHeaderCase: true,
+        headers: {'Sensitive': 'test', 'insensitive': 'test'},
+      ),
+    );
+    expect(headerMap['Sensitive'], 'test');
+    expect(headerMap['insensitive'], 'test');
+    headerMap.clear();
+
+    await dio.get(
+      '',
+      options: Options(
+        headers: {'Sensitive': 'test', 'insensitive': 'test'},
+      ),
+    );
+    expect(headerMap['sensitive'], 'test');
+    expect(headerMap['insensitive'], 'test');
+  });
 }
diff --git a/plugins/native_dio_adapter/CHANGELOG.md b/plugins/native_dio_adapter/CHANGELOG.md
index 7e9bdaf7e..53d3386b3 100644
--- a/plugins/native_dio_adapter/CHANGELOG.md
+++ b/plugins/native_dio_adapter/CHANGELOG.md
@@ -15,6 +15,7 @@
 - Bump `cronet_http` version.
 - Minimal required Dart version is now 3.1.
 - Minimal required Flutter version is now 3.13.0.
+- Allow case-sensitive header keys with the `preserveHeaderCase` flag through options.
 
 ## 1.0.0+2
 
diff --git a/plugins/native_dio_adapter/lib/src/conversion_layer_adapter.dart b/plugins/native_dio_adapter/lib/src/conversion_layer_adapter.dart
index a035f2c45..0e1904f0b 100644
--- a/plugins/native_dio_adapter/lib/src/conversion_layer_adapter.dart
+++ b/plugins/native_dio_adapter/lib/src/conversion_layer_adapter.dart
@@ -24,7 +24,7 @@ class ConversionLayerAdapter implements HttpClientAdapter {
   ) async {
     final request = await _fromOptionsAndStream(options, requestStream);
     final response = await client.send(request);
-    return response.toDioResponseBody();
+    return response.toDioResponseBody(options);
   }
 
   @override
@@ -41,7 +41,12 @@ class ConversionLayerAdapter implements HttpClientAdapter {
 
     request.headers.addAll(
       Map.fromEntries(
-        options.headers.entries.map((e) => MapEntry(e.key, e.value.toString())),
+        options.headers.entries.map(
+          (e) => MapEntry(
+            options.preserveHeaderCase ? e.key : e.key.toLowerCase(),
+            e.value.toString(),
+          ),
+        ),
       ),
     );
 
@@ -67,8 +72,13 @@ class ConversionLayerAdapter implements HttpClientAdapter {
 }
 
 extension on StreamedResponse {
-  ResponseBody toDioResponseBody() {
-    final dioHeaders = headers.entries.map((e) => MapEntry(e.key, [e.value]));
+  ResponseBody toDioResponseBody(RequestOptions options) {
+    final dioHeaders = headers.entries.map(
+      (e) => MapEntry(
+        options.preserveHeaderCase ? e.key : e.key.toLowerCase(),
+        [e.value],
+      ),
+    );
     return ResponseBody(
       stream.cast<Uint8List>(),
       statusCode,