diff --git a/example/README.md b/example/README.md
index bf6d1011..c1103c75 100644
--- a/example/README.md
+++ b/example/README.md
@@ -11,3 +11,10 @@ To run the Android JNI/JCA digest smoke test:
 flutter test integration_test/jni_digest_test.dart \
   -d emulator-name
 ```
+
+To run the Android JNI/JCA HMAC smoke test:
+
+```sh
+flutter test integration_test/jni_hmac_test.dart \
+  -d emulator-name
+```
diff --git a/example/integration_test/jni_hmac_test.dart b/example/integration_test/jni_hmac_test.dart
new file mode 100644
index 00000000..4b4184d4
--- /dev/null
+++ b/example/integration_test/jni_hmac_test.dart
@@ -0,0 +1,24 @@
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:integration_test/integration_test.dart';
+import 'package:webcrypto/webcrypto.dart';
+
+void main() {
+  IntegrationTestWidgetsFlutterBinding.ensureInitialized();
+
+  testWidgets('public API HMAC-SHA-256 signature matches known vector', (
+    _,
+  ) async {
+    final key = await HmacSecretKey.importRawKey(
+      List<int>.filled(20, 0x0b),
+      Hash.sha256,
+    );
+    final signature = await key.signBytes(utf8.encode('Hi There'));
+
+    expect(
+      base64Encode(signature),
+      'sDRMYdjbOFNcqK/OrwvxK4gdwgDJgz2nJuk3bC4yz/c=',
+    );
+  });
+}
diff --git a/lib/src/impl_jni/impl_jni.dart b/lib/src/impl_jni/impl_jni.dart
index 02b2a639..594d5794 100644
--- a/lib/src/impl_jni/impl_jni.dart
+++ b/lib/src/impl_jni/impl_jni.dart
@@ -20,11 +20,13 @@
 library;
 
 import 'dart:async';
+import 'dart:convert' show base64Url;
 import 'dart:typed_data';
 
 import 'package:jni/jni.dart' as jni;
 import 'package:webcrypto/src/impl_interface/impl_interface.dart';
 
+import '../jsonwebkey.dart' show JsonWebKey;
 import '../third_party/jca/generated_bindings.dart';
 
 part 'impl_jni.aescbc.dart';
diff --git a/lib/src/impl_jni/impl_jni.hmac.dart b/lib/src/impl_jni/impl_jni.hmac.dart
index 3c3a5bb2..f8cb11a5 100644
--- a/lib/src/impl_jni/impl_jni.hmac.dart
+++ b/lib/src/impl_jni/impl_jni.hmac.dart
@@ -1,4 +1,4 @@
-// Copyright 2020 Google LLC
+// Copyright 2026 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -14,6 +14,45 @@
 
 part of 'impl_jni.dart';
 
+_HashImpl _hmacHashFromHash(HashImpl hash) {
+  if (hash is _HashImpl) {
+    return hash;
+  }
+  throw AssertionError('Custom implementations of HashImpl are not supported.');
+}
+
+extension _HmacHashMetadata on _HashImpl {
+  String get _hmacJcaName {
+    return switch (_jcaName) {
+      'SHA-1' => 'HmacSHA1',
+      'SHA-256' => 'HmacSHA256',
+      'SHA-384' => 'HmacSHA384',
+      'SHA-512' => 'HmacSHA512',
+      _ => throw AssertionError('Unknown hash algorithm: $_jcaName'),
+    };
+  }
+
+  String get _hmacJwkAlg {
+    return switch (_jcaName) {
+      'SHA-1' => 'HS1',
+      'SHA-256' => 'HS256',
+      'SHA-384' => 'HS384',
+      'SHA-512' => 'HS512',
+      _ => throw AssertionError('Unknown hash algorithm: $_jcaName'),
+    };
+  }
+
+  int get _hmacDefaultLengthBits {
+    return switch (_jcaName) {
+      'SHA-1' => 160,
+      'SHA-256' => 256,
+      'SHA-384' => 384,
+      'SHA-512' => 512,
+      _ => throw AssertionError('Unknown hash algorithm: $_jcaName'),
+    };
+  }
+}
+
 final class _StaticHmacSecretKeyImpl implements StaticHmacSecretKeyImpl {
   const _StaticHmacSecretKeyImpl();
 
@@ -22,9 +61,10 @@ final class _StaticHmacSecretKeyImpl implements StaticHmacSecretKeyImpl {
     List<int> keyData,
     HashImpl hash, {
     int? length,
-  }) {
-    throw UnimplementedError('Not implemented');
-  }
+  }) async => _HmacSecretKeyImpl(
+    _asUint8ListZeroedToBitLength(keyData, length),
+    _hmacHashFromHash(hash),
+  );
 
   @override
   Future<HmacSecretKeyImpl> importJsonWebKey(
@@ -32,11 +72,133 @@ final class _StaticHmacSecretKeyImpl implements StaticHmacSecretKeyImpl {
     HashImpl hash, {
     int? length,
   }) {
-    throw UnimplementedError('Not implemented');
+    final h = _hmacHashFromHash(hash);
+    final key = JsonWebKey.fromJson(jwk);
+
+    void checkJwk(bool condition, String prop, String message) =>
+        _checkData(condition, 'JWK property "$prop" $message');
+
+    checkJwk(key.kty == 'oct', 'kty', 'must be "oct"');
+    checkJwk(key.k != null, 'k', 'must be present');
+    checkJwk(
+      key.use == null || key.use == 'sig',
+      'use',
+      'must be "sig", if present',
+    );
+    checkJwk(
+      key.alg == null || key.alg == h._hmacJwkAlg,
+      'alg',
+      'must be "${h._hmacJwkAlg}"',
+    );
+
+    final keyData = _jwkDecodeBase64UrlNoPadding(key.k!, 'k');
+    return importRawKey(keyData, hash, length: length);
   }
 
   @override
   Future<HmacSecretKeyImpl> generateKey(HashImpl hash, {int? length = 32}) {
-    throw UnimplementedError('Not implemented');
+    final h = _hmacHashFromHash(hash);
+    length ??= h._hmacDefaultLengthBits;
+    final keyData = _randomBytes((length + 7) ~/ 8);
+    return importRawKey(keyData, hash, length: length);
+  }
+}
+
+final class _HmacSecretKeyImpl implements HmacSecretKeyImpl {
+  _HmacSecretKeyImpl(this._keyData, this._hash);
+
+  final Uint8List _keyData;
+  final _HashImpl _hash;
+
+  @override
+  Future<Uint8List> signBytes(List<int> data) async {
+    return jni.using((arena) {
+      final mac = _createMac(arena);
+      final input = arena.copyToJByteArray(_asUint8List(data));
+      return _copyMacResult(arena, mac.doFinal$2(input));
+    });
+  }
+
+  @override
+  Future<Uint8List> signStream(Stream<List<int>> data) async {
+    final arena = jni.Arena();
+    try {
+      final mac = _createMac(arena);
+      final buffer = jni.JByteArray(_defaultChunkSize)..releasedBy(arena);
+      await for (final chunk in data) {
+        _updateMacWithChunk(mac, buffer, chunk);
+      }
+
+      return _copyMacResult(arena, mac.doFinal());
+    } finally {
+      arena.releaseAll();
+    }
+  }
+
+  @override
+  Future<bool> verifyBytes(List<int> signature, List<int> data) async {
+    return _verifySignature(await signBytes(data), signature);
+  }
+
+  @override
+  Future<bool> verifyStream(List<int> signature, Stream<List<int>> data) async {
+    return _verifySignature(await signStream(data), signature);
+  }
+
+  @override
+  Future<Uint8List> exportRawKey() async => Uint8List.fromList(_keyData);
+
+  @override
+  Future<Map<String, dynamic>> exportJsonWebKey() async {
+    return JsonWebKey(
+      kty: 'oct',
+      use: 'sig',
+      alg: _hash._hmacJwkAlg,
+      k: _jwkEncodeBase64UrlNoPadding(_keyData),
+    ).toJson();
+  }
+
+  Mac _createMac(jni.Arena arena) {
+    final algorithm = _hash._hmacJcaName.toJString()..releasedBy(arena);
+    final keyData = arena.copyToJByteArray(_keyData);
+    final key = SecretKeySpec(keyData, algorithm)..releasedBy(arena);
+
+    final mac = Mac.getInstance(algorithm);
+    if (mac == null) {
+      throw AssertionError('JCA Mac(${_hash._hmacJcaName}) returned null');
+    }
+    mac.releasedBy(arena);
+    mac.init(key);
+    return mac;
+  }
+
+  void _updateMacWithChunk(Mac mac, jni.JByteArray buffer, List<int> chunk) {
+    final bytes = _asUint8List(chunk);
+    var offset = 0;
+    while (offset < bytes.length) {
+      final remaining = bytes.length - offset;
+      final length = remaining < _defaultChunkSize
+          ? remaining
+          : _defaultChunkSize;
+      buffer.setRange(0, length, bytes, offset);
+      mac.update$2(buffer, 0, length);
+      offset += length;
+    }
+  }
+
+  Uint8List _copyMacResult(jni.Arena arena, jni.JByteArray? result) {
+    if (result == null) {
+      throw AssertionError('JCA Mac(${_hash._hmacJcaName}) returned null');
+    }
+    result.releasedBy(arena);
+    return result.copyToDartBytes();
+  }
+
+  bool _verifySignature(Uint8List computedMac, List<int> suppliedSignature) {
+    return jni.using((arena) {
+      final computed = arena.copyToJByteArray(computedMac);
+      final supplied = arena.copyToJByteArray(_asUint8List(suppliedSignature));
+      return MessageDigest.isEqual(computed, supplied);
+    });
   }
 }
diff --git a/lib/src/impl_jni/impl_jni.random.dart b/lib/src/impl_jni/impl_jni.random.dart
index 351ad0df..270931e9 100644
--- a/lib/src/impl_jni/impl_jni.random.dart
+++ b/lib/src/impl_jni/impl_jni.random.dart
@@ -18,7 +18,5 @@ final class _RandomImpl implements RandomImpl {
   const _RandomImpl();
 
   @override
-  void fillRandomBytes(TypedData destination) {
-    throw UnimplementedError('Not implemented');
-  }
+  void fillRandomBytes(TypedData destination) => _fillRandomBytes(destination);
 }
diff --git a/lib/src/impl_jni/impl_jni.utils.dart b/lib/src/impl_jni/impl_jni.utils.dart
index f5c64eaf..51354de2 100644
--- a/lib/src/impl_jni/impl_jni.utils.dart
+++ b/lib/src/impl_jni/impl_jni.utils.dart
@@ -14,15 +14,116 @@
 
 part of 'impl_jni.dart';
 
+const _defaultChunkSize = 4096;
+
+void _checkData(bool condition, String message) {
+  if (!condition) {
+    throw FormatException(message);
+  }
+}
+
+Uint8List _asUint8List(List<int> data) {
+  return data is Uint8List ? data : Uint8List.fromList(data);
+}
+
+Uint8List _asUint8ListZeroedToBitLength(List<int> data, [int? lengthInBits]) {
+  final bytes = Uint8List.fromList(data);
+  if (lengthInBits == null) {
+    return bytes;
+  }
+
+  final startFrom = lengthInBits ~/ 8;
+  var remainder = lengthInBits % 8;
+  for (var i = startFrom; i < bytes.length; i++) {
+    final mask = 0xff & (0xff << (8 - remainder));
+    bytes[i] = bytes[i] & mask;
+    remainder = 8;
+  }
+  return bytes;
+}
+
+Uint8List _jwkDecodeBase64UrlNoPadding(String unpadded, String prop) {
+  try {
+    final padded = unpadded.padRight(
+      unpadded.length + ((4 - (unpadded.length % 4)) % 4),
+      '=',
+    );
+    return base64Url.decode(padded);
+  } on FormatException {
+    throw FormatException(
+      'JWK property "$prop" is not url-safe base64 without padding',
+      unpadded,
+    );
+  }
+}
+
+String _jwkEncodeBase64UrlNoPadding(Uint8List data) {
+  final padded = base64Url.encode(data);
+  final paddingStart = padded.indexOf('=');
+  return paddingStart == -1 ? padded : padded.substring(0, paddingStart);
+}
+
+extension _JniArenaByteArray on jni.Arena {
+  jni.JByteArray copyToJByteArray(Uint8List data) {
+    return jni.JByteArray.of(data)..releasedBy(this);
+  }
+}
+
 extension _JByteArrayCopy on jni.JByteArray {
   /// Copies this JVM byte array into Dart-owned memory.
-  ///
-  /// `getRange` returns a typed list backed by a native buffer whose lifetime
-  /// is finalizer-driven. Copy once more before releasing the Java array so
-  /// webcrypto callers receive a normal Dart-managed [Uint8List].
   Uint8List copyToDartBytes() {
     final bytes = getRange(0, length);
-    final view = bytes.buffer.asUint8List(bytes.offsetInBytes, bytes.length);
+    final view = Uint8List.sublistView(bytes);
     return Uint8List.fromList(view);
   }
+
+  void copyRangeToDart(
+    Uint8List destination,
+    int destinationOffset,
+    int length,
+  ) {
+    final bytes = getRange(0, length);
+    final view = Uint8List.sublistView(bytes);
+    destination.setRange(destinationOffset, destinationOffset + length, view);
+  }
+}
+
+Uint8List _randomBytes(int length) {
+  final output = Uint8List(length);
+  _fillRandomBytes(output);
+  return output;
+}
+
+void _fillRandomBytes(TypedData destination) {
+  final output = destination.buffer.asUint8List(
+    destination.offsetInBytes,
+    destination.lengthInBytes,
+  );
+  if (output.isEmpty) {
+    return;
+  }
+
+  jni.using((arena) {
+    final random = SecureRandom()..releasedBy(arena);
+    final bufferLength = output.length < _defaultChunkSize
+        ? output.length
+        : _defaultChunkSize;
+    final fullBuffer = jni.JByteArray(bufferLength)..releasedBy(arena);
+
+    // TODO: Should revisit bulk input/output transfer helpers with JByteBuffer for
+    // JCA APIs that accept ByteBuffer directly. SecureRandom.nextBytes only accepts
+    // byte[].
+    var offset = 0;
+    while (offset < output.length) {
+      final remaining = output.length - offset;
+      final chunkLength = remaining < bufferLength ? remaining : bufferLength;
+      final bytes = chunkLength == bufferLength
+          ? fullBuffer
+          : (jni.JByteArray(chunkLength)..releasedBy(arena));
+
+      random.nextBytes(bytes);
+      bytes.copyRangeToDart(output, offset, chunkLength);
+      offset += chunkLength;
+    }
+  });
 }
diff --git a/test/impl_jni_hmac_test.dart b/test/impl_jni_hmac_test.dart
new file mode 100644
index 00000000..fe3ab357
--- /dev/null
+++ b/test/impl_jni_hmac_test.dart
@@ -0,0 +1,131 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+@TestOn('vm')
+library;
+
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:test/test.dart';
+import 'package:webcrypto/src/impl_ffi/impl_ffi.dart' as ffi_impl;
+import 'package:webcrypto/src/impl_jni/impl_jni.dart' as jni_impl;
+
+import 'src/jni_test_setup.dart'
+    if (dart.library.io) 'src/jni_test_setup_io.dart';
+
+void main() {
+  final skipReason = jniHelperSetupSkipReason;
+
+  setUpAll(() {
+    if (skipReason != null) {
+      return;
+    }
+
+    spawnJniForDesktopTests();
+  });
+
+  test('JCA HMAC-SHA-256 matches RFC 4231 test vector', () async {
+    final keyData = List<int>.filled(20, 0x0b);
+    final data = utf8.encode('Hi There');
+
+    final key = await jni_impl.webCryptImpl.hmacSecretKey.importRawKey(
+      keyData,
+      jni_impl.webCryptImpl.sha256,
+    );
+    final signature = await key.signBytes(data);
+
+    final ffiKey = await ffi_impl.webCryptImpl.hmacSecretKey.importRawKey(
+      keyData,
+      ffi_impl.webCryptImpl.sha256,
+    );
+    final ffiSignature = await ffiKey.signBytes(data);
+
+    expect(signature, ffiSignature);
+    expect(
+      base64Encode(signature),
+      'sDRMYdjbOFNcqK/OrwvxK4gdwgDJgz2nJuk3bC4yz/c=',
+    );
+    expect(await key.verifyBytes(signature, data), isTrue);
+
+    final badSignature = Uint8List.fromList(signature);
+    badSignature[badSignature.length - 1] ^= 0x01;
+    expect(await key.verifyBytes(badSignature, data), isFalse);
+    expect(await key.verifyBytes(signature.sublist(1), data), isFalse);
+  }, skip: skipReason);
+
+  test('JCA HMAC-SHA-512 stream signing matches FFI', () async {
+    final keyData = List<int>.filled(20, 0x0b);
+    final chunks = [utf8.encode('Hi '), utf8.encode('There')];
+
+    final key = await jni_impl.webCryptImpl.hmacSecretKey.importRawKey(
+      keyData,
+      jni_impl.webCryptImpl.sha512,
+    );
+    final signature = await key.signStream(Stream.fromIterable(chunks));
+
+    final ffiKey = await ffi_impl.webCryptImpl.hmacSecretKey.importRawKey(
+      keyData,
+      ffi_impl.webCryptImpl.sha512,
+    );
+    final ffiSignature = await ffiKey.signStream(Stream.fromIterable(chunks));
+
+    expect(signature, ffiSignature);
+    expect(
+      base64Encode(signature),
+      'h6p83qXvYZ1P8LQkGh1ssCN59OLOTsJ4etCzBUXhfN7aqDO31rinAgOLJ06uo/Tk'
+      'vp2RTuth8XAuaWwgOhJoVA==',
+    );
+    expect(
+      await key.verifyStream(signature, Stream.fromIterable(chunks)),
+      isTrue,
+    );
+  }, skip: skipReason);
+
+  test('JCA HMAC exports and imports JSON Web Keys', () async {
+    final keyData = utf8.encode('sample-hmac-key');
+    final key = await jni_impl.webCryptImpl.hmacSecretKey.importRawKey(
+      keyData,
+      jni_impl.webCryptImpl.sha384,
+    );
+
+    final jwk = await key.exportJsonWebKey();
+    expect(jwk['kty'], 'oct');
+    expect(jwk['use'], 'sig');
+    expect(jwk['alg'], 'HS384');
+    expect(jwk['k'], 'c2FtcGxlLWhtYWMta2V5');
+
+    final imported = await jni_impl.webCryptImpl.hmacSecretKey.importJsonWebKey(
+      jwk,
+      jni_impl.webCryptImpl.sha384,
+    );
+    final data = utf8.encode('message');
+
+    expect(await imported.signBytes(data), await key.signBytes(data));
+  }, skip: skipReason);
+
+  test('JCA HMAC generateKey supports non-byte-aligned key lengths', () async {
+    final key = await jni_impl.webCryptImpl.hmacSecretKey.generateKey(
+      jni_impl.webCryptImpl.sha512,
+      length: 37,
+    );
+
+    final keyData = await key.exportRawKey();
+    expect(keyData, hasLength(5));
+    expect(keyData.last & 0x07, 0);
+
+    final signature = await key.signBytes(utf8.encode('message'));
+    expect(await key.verifyBytes(signature, utf8.encode('message')), isTrue);
+  }, skip: skipReason);
+}