diff --git a/src/Symfony/Component/Notifier/Bridge/Slack/CHANGELOG.md b/src/Symfony/Component/Notifier/Bridge/Slack/CHANGELOG.md
index 650a09bc1eec8..bb2c7a9ecc0a0 100644
--- a/src/Symfony/Component/Notifier/Bridge/Slack/CHANGELOG.md
+++ b/src/Symfony/Component/Notifier/Bridge/Slack/CHANGELOG.md
@@ -6,6 +6,7 @@ CHANGELOG
 
  * Check for maximum number of buttons in Slack action block
  * Add HeaderBlock
+ * Slack access tokens needs to start with "xox" (see https://api.slack.com/authentication/token-types)
 
 5.2.0
 -----
diff --git a/src/Symfony/Component/Notifier/Bridge/Slack/SlackTransport.php b/src/Symfony/Component/Notifier/Bridge/Slack/SlackTransport.php
index dba82100503bb..c877d706d2635 100644
--- a/src/Symfony/Component/Notifier/Bridge/Slack/SlackTransport.php
+++ b/src/Symfony/Component/Notifier/Bridge/Slack/SlackTransport.php
@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Notifier\Bridge\Slack;
 
+use Symfony\Component\Notifier\Exception\InvalidArgumentException;
 use Symfony\Component\Notifier\Exception\LogicException;
 use Symfony\Component\Notifier\Exception\TransportException;
 use Symfony\Component\Notifier\Exception\UnsupportedMessageTypeException;
@@ -35,6 +36,10 @@ final class SlackTransport extends AbstractTransport
 
     public function __construct(string $accessToken, string $channel = null, HttpClientInterface $client = null, EventDispatcherInterface $dispatcher = null)
     {
+        if (!preg_match('/^xox(b-|p-|a-2)/', $accessToken)) {
+            throw new InvalidArgumentException('A valid Slack token needs to start with "xoxb-", "xoxp-" or "xoxa-2". See https://api.slack.com/authentication/token-types for further information.');
+        }
+
         $this->accessToken = $accessToken;
         $this->chatChannel = $channel;
         $this->client = $client;
diff --git a/src/Symfony/Component/Notifier/Bridge/Slack/Tests/SlackTransportFactoryTest.php b/src/Symfony/Component/Notifier/Bridge/Slack/Tests/SlackTransportFactoryTest.php
index 39c5396179455..74c8147cb3ff1 100644
--- a/src/Symfony/Component/Notifier/Bridge/Slack/Tests/SlackTransportFactoryTest.php
+++ b/src/Symfony/Component/Notifier/Bridge/Slack/Tests/SlackTransportFactoryTest.php
@@ -24,7 +24,7 @@ public function testCreateWithDsn()
     {
         $factory = $this->createFactory();
 
-        $transport = $factory->create(Dsn::fromString('slack://testUser@host.test/?channel=testChannel'));
+        $transport = $factory->create(Dsn::fromString('slack://xoxb-TestUser@host.test/?channel=testChannel'));
 
         $this->assertSame('slack://host.test?channel=testChannel', (string) $transport);
     }
@@ -33,7 +33,7 @@ public function testCreateWithDsnWithoutPath()
     {
         $factory = $this->createFactory();
 
-        $transport = $factory->create(Dsn::fromString('slack://testUser@host.test?channel=testChannel'));
+        $transport = $factory->create(Dsn::fromString('slack://xoxb-TestUser@host.test?channel=testChannel'));
 
         $this->assertSame('slack://host.test?channel=testChannel', (string) $transport);
     }
diff --git a/src/Symfony/Component/Notifier/Bridge/Slack/Tests/SlackTransportTest.php b/src/Symfony/Component/Notifier/Bridge/Slack/Tests/SlackTransportTest.php
index b97f59c2a74de..6e033c07b4a7a 100644
--- a/src/Symfony/Component/Notifier/Bridge/Slack/Tests/SlackTransportTest.php
+++ b/src/Symfony/Component/Notifier/Bridge/Slack/Tests/SlackTransportTest.php
@@ -15,6 +15,7 @@
 use Symfony\Component\HttpClient\MockHttpClient;
 use Symfony\Component\Notifier\Bridge\Slack\SlackOptions;
 use Symfony\Component\Notifier\Bridge\Slack\SlackTransport;
+use Symfony\Component\Notifier\Exception\InvalidArgumentException;
 use Symfony\Component\Notifier\Exception\LogicException;
 use Symfony\Component\Notifier\Exception\TransportException;
 use Symfony\Component\Notifier\Exception\UnsupportedMessageTypeException;
@@ -31,15 +32,23 @@ public function testToStringContainsProperties()
     {
         $channel = 'test Channel'; // invalid channel name to test url encoding of the channel
 
-        $transport = new SlackTransport('testToken', $channel, $this->createMock(HttpClientInterface::class));
+        $transport = new SlackTransport('xoxb-TestToken', $channel, $this->createMock(HttpClientInterface::class));
         $transport->setHost('host.test');
 
         $this->assertSame('slack://host.test?channel=test+Channel', (string) $transport);
     }
 
+    public function testInstatiatingWithAnInvalidSlackTokenThrowsInvalidArgumentException()
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('A valid Slack token needs to start with "xoxb-", "xoxp-" or "xoxa-2". See https://api.slack.com/authentication/token-types for further information.');
+
+        new SlackTransport('token', 'testChannel', $this->createMock(HttpClientInterface::class));
+    }
+
     public function testSupportsChatMessage()
     {
-        $transport = new SlackTransport('testToken', 'testChannel', $this->createMock(HttpClientInterface::class));
+        $transport = new SlackTransport('xoxb-TestToken', 'testChannel', $this->createMock(HttpClientInterface::class));
 
         $this->assertTrue($transport->supports(new ChatMessage('testChatMessage')));
         $this->assertFalse($transport->supports($this->createMock(MessageInterface::class)));
@@ -47,7 +56,7 @@ public function testSupportsChatMessage()
 
     public function testSendNonChatMessageThrowsLogicException()
     {
-        $transport = new SlackTransport('testToken', 'testChannel', $this->createMock(HttpClientInterface::class));
+        $transport = new SlackTransport('xoxb-TestToken', 'testChannel', $this->createMock(HttpClientInterface::class));
 
         $this->expectException(UnsupportedMessageTypeException::class);
 
@@ -70,7 +79,7 @@ public function testSendWithEmptyArrayResponseThrows()
             return $response;
         });
 
-        $transport = new SlackTransport('testToken', 'testChannel', $client);
+        $transport = new SlackTransport('xoxb-TestToken', 'testChannel', $client);
 
         $transport->send(new ChatMessage('testMessage'));
     }
@@ -93,14 +102,14 @@ public function testSendWithErrorResponseThrows()
             return $response;
         });
 
-        $transport = new SlackTransport('testToken', 'testChannel', $client);
+        $transport = new SlackTransport('xoxb-TestToken', 'testChannel', $client);
 
         $transport->send(new ChatMessage('testMessage'));
     }
 
     public function testSendWithOptions()
     {
-        $token = 'testToken';
+        $token = 'xoxb-TestToken';
         $channel = 'testChannel';
         $message = 'testMessage';
 
@@ -129,7 +138,7 @@ public function testSendWithOptions()
 
     public function testSendWithNotification()
     {
-        $token = 'testToken';
+        $token = 'xoxb-TestToken';
         $channel = 'testChannel';
         $message = 'testMessage';
 
@@ -172,14 +181,14 @@ public function testSendWithInvalidOptions()
             return $this->createMock(ResponseInterface::class);
         });
 
-        $transport = new SlackTransport('testToken', 'testChannel', $client);
+        $transport = new SlackTransport('xoxb-TestToken', 'testChannel', $client);
 
         $transport->send(new ChatMessage('testMessage', $this->createMock(MessageOptionsInterface::class)));
     }
 
     public function testSendWith200ResponseButNotOk()
     {
-        $token = 'testToken';
+        $token = 'xoxb-TestToken';
         $channel = 'testChannel';
         $message = 'testMessage';