diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6c8e9e6c..fcde1f9b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased](https://github.com/kbsali/php-redmine-api/compare/v2.4.0...v2.x)
 
+### Added
+
+- Added support for updating groups.
+
 ### Changed
 
 - The last response is saved in `Redmine\Api\AbstractApi` to prevent race conditions with `Redmine\Client\Client` implementations.
diff --git a/docs/usage.md b/docs/usage.md
index f0c5ac4a..d195eccd 100644
--- a/docs/usage.md
+++ b/docs/usage.md
@@ -489,6 +489,25 @@ $client->getApi('group')->removeUser($groupId, $userId);
 $client->getApi('group')->create([
     'name' => 'asdf',
     'user_ids' => [1, 2],
+    'custom_fields' => [
+        [
+            'id' => 123,
+            'name' => 'cf_name',
+            'value' => 'cf_value',
+        ],
+    ],
+]);
+$client->getApi('group')->update($groupId, [
+    'name' => 'asdf',
+    // Note: you can only add users this way; use removeUser to remove a user
+    'user_ids' => [1, 2],
+    'custom_fields' => [
+        [
+            'id' => 123,
+            'name' => 'cf_name',
+            'value' => 'cf_value',
+        ],
+    ],
 ]);
 
 // ----------------------------
diff --git a/src/Redmine/Api/Group.php b/src/Redmine/Api/Group.php
index 206dcc62..4ee6442d 100644
--- a/src/Redmine/Api/Group.php
+++ b/src/Redmine/Api/Group.php
@@ -8,6 +8,7 @@
 use Redmine\Exception\UnexpectedResponseException;
 use Redmine\Serializer\PathSerializer;
 use Redmine\Serializer\XmlSerializer;
+use SimpleXMLElement;
 
 /**
  * Handling of groups.
@@ -101,7 +102,7 @@ public function listing($forceUpdate = false)
      *
      * @throws MissingParameterException Missing mandatory parameters
      *
-     * @return string|false
+     * @return string|SimpleXMLElement|false
      */
     public function create(array $params = [])
     {
@@ -124,17 +125,28 @@ public function create(array $params = [])
     }
 
     /**
+     * Updates a group.
+     *
      * NOT DOCUMENTED in Redmine's wiki.
      *
      * @see http://www.redmine.org/projects/redmine/wiki/Rest_Groups#PUT
      *
-     * @param int $id
+     * @param int $id the group id
      *
-     * @throws Exception Not implemented
+     * @return string empty string
      */
-    public function update($id, array $params = [])
+    public function update(int $id, array $params = [])
     {
-        throw new \Exception('Not implemented');
+        $defaults = [
+            'name' => null,
+            'user_ids' => null,
+        ];
+        $params = $this->sanitizeParams($defaults, $params);
+
+        return $this->put(
+            '/groups/' . $id . '.xml',
+            XmlSerializer::createFromArray(['group' => $params])->getEncoded()
+        );
     }
 
     /**
diff --git a/tests/Integration/GroupXmlTest.php b/tests/Integration/GroupXmlTest.php
index 3f7af662..6b64ce41 100644
--- a/tests/Integration/GroupXmlTest.php
+++ b/tests/Integration/GroupXmlTest.php
@@ -2,7 +2,6 @@
 
 namespace Redmine\Tests\Integration;
 
-use Exception;
 use PHPUnit\Framework\TestCase;
 use Redmine\Exception\MissingParameterException;
 use Redmine\Tests\Fixtures\MockClient;
@@ -48,15 +47,30 @@ public function testCreateComplex()
         );
     }
 
-    public function testUpdateNotImplemented()
+    public function testUpdateComplex()
     {
         /** @var \Redmine\Api\Group */
         $api = MockClient::create()->getApi('group');
-        $this->assertInstanceOf('Redmine\Api\Group', $api);
-
-        $this->expectException(Exception::class);
-        $this->expectExceptionMessage('Not implemented');
+        $res = $api->update(5, [
+            'name' => 'Developers',
+            'user_ids' => [3, 5],
+        ]);
+        $response = json_decode($res, true);
 
-        $api->update(1);
+        $this->assertEquals('PUT', $response['method']);
+        $this->assertEquals('/groups/5.xml', $response['path']);
+        $this->assertXmlStringEqualsXmlString(
+            <<< XML
+            <?xml version="1.0"?>
+            <group>
+                <name>Developers</name>
+                <user_ids type="array">
+                    <user_id>3</user_id>
+                    <user_id>5</user_id>
+                </user_ids>
+            </group>
+            XML,
+            $response['data']
+        );
     }
 }
diff --git a/tests/Unit/Api/Group/CreateTest.php b/tests/Unit/Api/Group/CreateTest.php
new file mode 100644
index 00000000..7e34cdce
--- /dev/null
+++ b/tests/Unit/Api/Group/CreateTest.php
@@ -0,0 +1,137 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Redmine\Tests\Unit\Api\Group;
+
+use PHPUnit\Framework\TestCase;
+use Redmine\Api\Group;
+use Redmine\Exception\MissingParameterException;
+use Redmine\Http\HttpClient;
+use Redmine\Http\Response;
+use SimpleXMLElement;
+
+/**
+ * @covers \Redmine\Api\Group::create
+ */
+class CreateTest extends TestCase
+{
+    public function testCreateWithNameCreatesGroup()
+    {
+        $client = $this->createMock(HttpClient::class);
+        $client->expects($this->exactly(1))
+            ->method('request')
+            ->willReturnCallback(function (string $method, string $path, string $body = '') {
+                $this->assertSame('POST', $method);
+                $this->assertSame('/groups.xml', $path);
+                $this->assertXmlStringEqualsXmlString('<?xml version="1.0"?><group><name>Group Name</name></group>', $body);
+
+                return $this->createConfiguredMock(
+                    Response::class,
+                    [
+                        'getContentType' => 'application/xml',
+                        'getBody' => '<?xml version="1.0"?><group></group>',
+                    ]
+                );
+            });
+
+        // Create the object under test
+        $api = new Group($client);
+
+        // Perform the tests
+        $xmlElement = $api->create(['name' => 'Group Name']);
+
+        $this->assertInstanceOf(SimpleXMLElement::class, $xmlElement);
+        $this->assertXmlStringEqualsXmlString(
+            '<?xml version="1.0"?><group></group>',
+            $xmlElement->asXml(),
+        );
+    }
+
+    public function testCreateWithNameAndUserIdsCreatesGroup()
+    {
+        $client = $this->createMock(HttpClient::class);
+        $client->expects($this->exactly(1))
+            ->method('request')
+            ->willReturnCallback(function (string $method, string $path, string $body = '') {
+                $this->assertSame('POST', $method);
+                $this->assertSame('/groups.xml', $path);
+                $this->assertXmlStringEqualsXmlString('<?xml version="1.0"?><group><name>Group Name</name><user_ids type="array"><user_id>1</user_id><user_id>2</user_id><user_id>3</user_id></user_ids></group>', $body);
+
+                return $this->createConfiguredMock(
+                    Response::class,
+                    [
+                        'getContentType' => 'application/xml',
+                        'getBody' => '<?xml version="1.0"?><group></group>',
+                    ]
+                );
+            });
+
+        // Create the object under test
+        $api = new Group($client);
+
+        // Perform the tests
+        $xmlElement = $api->create(['name' => 'Group Name', 'user_ids' => [1, 2, 3]]);
+
+        $this->assertInstanceOf(SimpleXMLElement::class, $xmlElement);
+        $this->assertXmlStringEqualsXmlString(
+            '<?xml version="1.0"?><group></group>',
+            $xmlElement->asXml(),
+        );
+    }
+
+    public function testCreateWithNameAndCustomFieldsCreatesGroup()
+    {
+        $client = $this->createMock(HttpClient::class);
+        $client->expects($this->exactly(1))
+            ->method('request')
+            ->willReturnCallback(function (string $method, string $path, string $body = '') {
+                $this->assertSame('POST', $method);
+                $this->assertSame('/groups.xml', $path);
+                $this->assertXmlStringEqualsXmlString('<?xml version="1.0"?><group><name>Group Name</name><custom_fields type="array"><custom_field id="1"><value>5</value></custom_field></custom_fields></group>', $body);
+
+                return $this->createConfiguredMock(
+                    Response::class,
+                    [
+                        'getContentType' => 'application/xml',
+                        'getBody' => '<?xml version="1.0"?><group></group>',
+                    ]
+                );
+            });
+
+        // Create the object under test
+        $api = new Group($client);
+
+        // Perform the tests
+        $xmlElement = $api->create([
+            'name' => 'Group Name',
+            'custom_fields' => [
+                ['id' => 1, 'value' => 5],
+            ],
+        ]);
+
+        $this->assertInstanceOf(SimpleXMLElement::class, $xmlElement);
+        $this->assertXmlStringEqualsXmlString(
+            '<?xml version="1.0"?><group></group>',
+            $xmlElement->asXml(),
+        );
+    }
+
+    public function testCreateThrowsExceptionIfNameIsMissing()
+    {
+        // Test values
+        $postParameter = [];
+
+        // Create the used mock objects
+        $client = $this->createMock(HttpClient::class);
+
+        // Create the object under test
+        $api = new Group($client);
+
+        $this->expectException(MissingParameterException::class);
+        $this->expectExceptionMessage('Theses parameters are mandatory: `name`');
+
+        // Perform the tests
+        $api->create($postParameter);
+    }
+}
diff --git a/tests/Unit/Api/Group/UpdateTest.php b/tests/Unit/Api/Group/UpdateTest.php
new file mode 100644
index 00000000..08bdaed6
--- /dev/null
+++ b/tests/Unit/Api/Group/UpdateTest.php
@@ -0,0 +1,105 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Redmine\Tests\Unit\Api\Group;
+
+use PHPUnit\Framework\TestCase;
+use Redmine\Api\Group;
+use Redmine\Http\HttpClient;
+use Redmine\Http\Response;
+use SimpleXMLElement;
+
+/**
+ * @covers \Redmine\Api\Group::update
+ */
+class UpdateTest extends TestCase
+{
+    public function testUpdateWithNameUpdatesGroup()
+    {
+        $client = $this->createMock(HttpClient::class);
+        $client->expects($this->exactly(1))
+            ->method('request')
+            ->willReturnCallback(function (string $method, string $path, string $body = '') {
+                $this->assertSame('PUT', $method);
+                $this->assertSame('/groups/1.xml', $path);
+                $this->assertXmlStringEqualsXmlString('<?xml version="1.0"?><group><name>Group Name</name></group>', $body);
+
+                return $this->createConfiguredMock(
+                    Response::class,
+                    [
+                        'getContentType' => 'application/xml',
+                        'getBody' => '',
+                    ]
+                );
+            });
+
+        // Create the object under test
+        $api = new Group($client);
+
+        // Perform the tests
+        $return = $api->update(1, ['name' => 'Group Name']);
+
+        $this->assertSame('', $return);
+    }
+
+    public function testUpdateWithUserIdsUpdatesGroup()
+    {
+        $client = $this->createMock(HttpClient::class);
+        $client->expects($this->exactly(1))
+            ->method('request')
+            ->willReturnCallback(function (string $method, string $path, string $body = '') {
+                $this->assertSame('PUT', $method);
+                $this->assertSame('/groups/1.xml', $path);
+                $this->assertXmlStringEqualsXmlString('<?xml version="1.0"?><group><user_ids type="array"><user_id>1</user_id><user_id>2</user_id><user_id>3</user_id></user_ids></group>', $body);
+
+                return $this->createConfiguredMock(
+                    Response::class,
+                    [
+                        'getContentType' => 'application/xml',
+                        'getBody' => '',
+                    ]
+                );
+            });
+
+        // Create the object under test
+        $api = new Group($client);
+
+        // Perform the tests
+        $return = $api->update(1, ['user_ids' => [1, 2, 3]]);
+
+        $this->assertSame('', $return);
+    }
+
+    public function testUpdateWithCustomFieldsUpdatesGroup()
+    {
+        $client = $this->createMock(HttpClient::class);
+        $client->expects($this->exactly(1))
+            ->method('request')
+            ->willReturnCallback(function (string $method, string $path, string $body = '') {
+                $this->assertSame('PUT', $method);
+                $this->assertSame('/groups/1.xml', $path);
+                $this->assertXmlStringEqualsXmlString('<?xml version="1.0"?><group><custom_fields type="array"><custom_field id="1"><value>5</value></custom_field></custom_fields></group>', $body);
+
+                return $this->createConfiguredMock(
+                    Response::class,
+                    [
+                        'getContentType' => 'application/xml',
+                        'getBody' => '',
+                    ]
+                );
+            });
+
+        // Create the object under test
+        $api = new Group($client);
+
+        // Perform the tests
+        $return = $api->update(1, [
+            'custom_fields' => [
+                ['id' => 1, 'value' => 5],
+            ],
+        ]);
+
+        $this->assertSame('', $return);
+    }
+}
diff --git a/tests/Unit/Api/GroupTest.php b/tests/Unit/Api/GroupTest.php
index fa5fc56a..8188a499 100644
--- a/tests/Unit/Api/GroupTest.php
+++ b/tests/Unit/Api/GroupTest.php
@@ -1,11 +1,12 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Redmine\Tests\Unit\Api;
 
 use PHPUnit\Framework\TestCase;
 use Redmine\Api\Group;
 use Redmine\Client\Client;
-use Redmine\Exception\MissingParameterException;
 use Redmine\Tests\Fixtures\MockClient;
 
 /**
@@ -341,73 +342,6 @@ public function testRemoveCallsDelete()
         $this->assertSame($expectedReturn, $api->remove(5));
     }
 
-    /**
-     * Test create().
-     *
-     * @covers ::create
-     * @covers ::post
-     * @test
-     */
-    public function testCreateCallsPost()
-    {
-        // Test values
-        $response = 'API Response';
-        $postParameter = [
-            'name' => 'Group Name',
-        ];
-
-        // Create the used mock objects
-        $client = $this->createMock(Client::class);
-        $client->expects($this->once())
-            ->method('requestPost')
-            ->with(
-                $this->logicalAnd(
-                    $this->stringStartsWith('/groups'),
-                    $this->logicalXor(
-                        $this->stringEndsWith('.json'),
-                        $this->stringEndsWith('.xml')
-                    )
-                ),
-                $this->stringContains('<group><name>Group Name</name></group>')
-            )
-            ->willReturn(true);
-        $client->expects($this->exactly(1))
-            ->method('getLastResponseBody')
-            ->willReturn($response);
-
-        // Create the object under test
-        $api = new Group($client);
-
-        // Perform the tests
-        $this->assertSame($response, $api->create($postParameter));
-    }
-
-    /**
-     * Test create().
-     *
-     * @covers ::create
-     * @covers ::post
-     *
-     * @test
-     */
-    public function testCreateThrowsExceptionIfNameIsMissing()
-    {
-        // Test values
-        $postParameter = [];
-
-        // Create the used mock objects
-        $client = $this->createMock(Client::class);
-
-        // Create the object under test
-        $api = new Group($client);
-
-        $this->expectException(MissingParameterException::class);
-        $this->expectExceptionMessage('Theses parameters are mandatory: `name`');
-
-        // Perform the tests
-        $api->create($postParameter);
-    }
-
     /**
      * Test removeUser().
      *