diff --git a/CHANGELOG.md b/CHANGELOG.md
index 790906270..4fc16808a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,12 @@ Please update your remote URL if you have forked or cloned the repository.
 
 ## Unreleased
 
+## Messaging
+
+* FCM `403 PERMISSION_DENIED` responses with the error code `SENDER_ID_MISMATCH` are now
+  converted to a dedicated `Kreait\Firebase\Exception\Messaging\SenderIdMismatch` exception instead of
+  the more generic `AuthenticationError`.
+
 ## 8.4.0 - 2026-08-05
 
 * Added support for `guzzlehttp/guzzle:^8.0`, `guzzlehttp/psr7:^3.0` and `guzzlehttp/promises:^3.0`.
diff --git a/docs/cloud-messaging.rst b/docs/cloud-messaging.rst
index 77fb5915b..97165870d 100644
--- a/docs/cloud-messaging.rst
+++ b/docs/cloud-messaging.rst
@@ -779,6 +779,27 @@ syntactically correct, this usually has one of the following reasons:
         echo $e->token();
     }
 
+Mismatched registration tokens
+------------------------------
+
+If a registration token is registered to a *different* Firebase project than the project
+you are using to send the message, the FCM API rejects the message with a
+``SENDER_ID_MISMATCH`` error. Like an unknown token, this is a permanent failure for
+this token - it will never be reachable from your project, so it should be removed
+from your list of message targets. In contrast to other authentication errors, it does
+not indicate a problem with your project's credentials.
+
+.. code-block:: php
+
+    use Kreait\Firebase\Exception\Messaging\SenderIdMismatch;
+
+    try {
+        $messaging->send($message);
+    } catch (SenderIdMismatch $e) {
+        echo $e->getMessage();
+        print_r($e->errors());
+    }
+
 Quota exceeded
 --------------
 
@@ -836,6 +857,8 @@ Error handling example
         $messaging->send($message);
     } catch (MessagingErrors\NotFound $e) {
         echo 'The target device could not be found.';
+    } catch (MessagingErrors\SenderIdMismatch $e) {
+        echo 'The target device belongs to a different Firebase project.';
     } catch (MessagingErrors\InvalidMessage $e) {
         echo 'The given message is malformatted.';
     } catch (MessagingErrors\ServerUnavailable $e) {
diff --git a/src/Exception/Messaging/SenderIdMismatch.php b/src/Exception/Messaging/SenderIdMismatch.php
new file mode 100644
index 000000000..cb4524359
--- /dev/null
+++ b/src/Exception/Messaging/SenderIdMismatch.php
@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Kreait\Firebase\Exception\Messaging;
+
+use Kreait\Firebase\Exception\HasErrors;
+use Kreait\Firebase\Exception\MessagingException;
+use Kreait\Firebase\Exception\RuntimeException;
+
+/**
+ * The registration token is registered to a different Firebase project than the
+ * project the message is sent from. This is a permanent, per-token failure: the
+ * token should be treated like an unregistered token and removed from the list
+ * of message targets.
+ */
+final class SenderIdMismatch extends RuntimeException implements MessagingException
+{
+    use HasErrors;
+
+    /**
+     * @internal
+     *
+     * @param array<mixed> $errors
+     */
+    public function withErrors(array $errors): self
+    {
+        $new = new self(message: $this->getMessage(), previous: $this->getPrevious());
+        $new->errors = $errors;
+
+        return $new;
+    }
+}
diff --git a/src/Exception/MessagingApiExceptionConverter.php b/src/Exception/MessagingApiExceptionConverter.php
index e512626e5..449880ef8 100644
--- a/src/Exception/MessagingApiExceptionConverter.php
+++ b/src/Exception/MessagingApiExceptionConverter.php
@@ -15,6 +15,7 @@
 use Kreait\Firebase\Exception\Messaging\MessagingError;
 use Kreait\Firebase\Exception\Messaging\NotFound;
 use Kreait\Firebase\Exception\Messaging\QuotaExceeded;
+use Kreait\Firebase\Exception\Messaging\SenderIdMismatch;
 use Kreait\Firebase\Exception\Messaging\ServerError;
 use Kreait\Firebase\Exception\Messaging\ServerUnavailable;
 use Kreait\Firebase\Http\ErrorResponseParser;
@@ -23,6 +24,7 @@
 use Psr\Http\Message\ResponseInterface;
 use Throwable;
 
+use function is_array;
 use function is_numeric;
 
 /**
@@ -74,11 +76,17 @@ public function convertResponse(ResponseInterface $response, ?Throwable $previou
                 break;
 
             case StatusCode::STATUS_UNAUTHORIZED:
-            case StatusCode::STATUS_FORBIDDEN:
                 $convertedError = new AuthenticationError($message, previous: $previous);
 
                 break;
 
+            case StatusCode::STATUS_FORBIDDEN:
+                $convertedError = $this->isSenderIdMismatch($errors, $message)
+                    ? new SenderIdMismatch($message, previous: $previous)
+                    : new AuthenticationError($message, previous: $previous);
+
+                break;
+
             case StatusCode::STATUS_NOT_FOUND:
                 $convertedError = new NotFound($message, previous: $previous);
 
@@ -148,6 +156,26 @@ private function convertGuzzleRequestException(RequestException $e): MessagingEx
         return new MessagingError(message: $e->getMessage(), previous: $e);
     }
 
+    /**
+     * @see https://firebase.google.com/docs/reference/fcm/rest/v1/ErrorCode
+     *
+     * @param array<mixed> $errors
+     */
+    private function isSenderIdMismatch(array $errors, string $message): bool
+    {
+        $details = $errors['error']['details'] ?? [];
+
+        if (is_array($details)) {
+            foreach ($details as $detail) {
+                if (is_array($detail) && ($detail['errorCode'] ?? null) === 'SENDER_ID_MISMATCH') {
+                    return true;
+                }
+            }
+        }
+
+        return mb_strtolower($message) === 'senderid mismatch';
+    }
+
     private function getRetryAfter(ResponseInterface $response): ?DateTimeImmutable
     {
         $retryAfter = $response->getHeaderLine('Retry-After');
diff --git a/src/Messaging/SendReport.php b/src/Messaging/SendReport.php
index 5842c62e7..4d8a2045f 100644
--- a/src/Messaging/SendReport.php
+++ b/src/Messaging/SendReport.php
@@ -6,6 +6,7 @@
 
 use Kreait\Firebase\Exception\Messaging\InvalidMessage;
 use Kreait\Firebase\Exception\Messaging\NotFound;
+use Kreait\Firebase\Exception\Messaging\SenderIdMismatch;
 use Kreait\Firebase\Exception\MessagingException;
 
 use function preg_match;
@@ -75,7 +76,8 @@ public function messageWasInvalid(): bool
 
     public function messageWasSentToUnknownToken(): bool
     {
-        return $this->error instanceof NotFound;
+        return $this->error instanceof NotFound
+            || $this->error instanceof SenderIdMismatch;
     }
 
     /**
diff --git a/tests/Unit/Exception/MessagingApiExceptionConverterTest.php b/tests/Unit/Exception/MessagingApiExceptionConverterTest.php
index 7c10b6eae..27a4a9da0 100644
--- a/tests/Unit/Exception/MessagingApiExceptionConverterTest.php
+++ b/tests/Unit/Exception/MessagingApiExceptionConverterTest.php
@@ -18,6 +18,7 @@
 use Kreait\Firebase\Exception\Messaging\MessagingError;
 use Kreait\Firebase\Exception\Messaging\NotFound;
 use Kreait\Firebase\Exception\Messaging\QuotaExceeded;
+use Kreait\Firebase\Exception\Messaging\SenderIdMismatch;
 use Kreait\Firebase\Exception\Messaging\ServerError;
 use Kreait\Firebase\Exception\Messaging\ServerUnavailable;
 use Kreait\Firebase\Exception\MessagingApiExceptionConverter;
@@ -107,6 +108,56 @@ public static function createRequestException(int $code, string $identifier): Re
         );
     }
 
+    public function testItConvertsASenderIdMismatchResponseIdentifiedByItsErrorCode(): void
+    {
+        // Error shape as returned by the FCM HTTP v1 API
+        // https://firebase.google.com/docs/reference/fcm/rest/v1/ErrorCode
+        $response = new Response(403, [], Json::encode([
+            'error' => [
+                'code' => 403,
+                'message' => 'SenderId mismatch',
+                'status' => 'PERMISSION_DENIED',
+                'details' => [
+                    [
+                        '@type' => 'type.googleapis.com/google.firebase.fcm.v1.FcmError',
+                        'errorCode' => 'SENDER_ID_MISMATCH',
+                    ],
+                ],
+            ],
+        ]));
+
+        $converted = $this->converter->convertResponse($response);
+
+        $this->assertInstanceOf(SenderIdMismatch::class, $converted);
+        $this->assertNotEmpty($converted->errors());
+    }
+
+    public function testItConvertsASenderIdMismatchResponseIdentifiedByItsMessage(): void
+    {
+        $response = new Response(403, [], Json::encode([
+            'error' => [
+                'code' => 403,
+                'message' => 'SenderId mismatch',
+                'status' => 'PERMISSION_DENIED',
+            ],
+        ]));
+
+        $this->assertInstanceOf(SenderIdMismatch::class, $this->converter->convertResponse($response));
+    }
+
+    public function testItConvertsOtherForbiddenResponsesToAuthenticationErrors(): void
+    {
+        $response = new Response(403, [], Json::encode([
+            'error' => [
+                'code' => 403,
+                'message' => 'The caller does not have permission',
+                'status' => 'PERMISSION_DENIED',
+            ],
+        ]));
+
+        $this->assertInstanceOf(AuthenticationError::class, $this->converter->convertResponse($response));
+    }
+
     public function testItKnowsWhenToRetryAfterWithSeconds(): void
     {
         $response = new Response(429, ['Retry-After' => '60']);
diff --git a/tests/Unit/Messaging/MulticastSendReportTest.php b/tests/Unit/Messaging/MulticastSendReportTest.php
new file mode 100644
index 000000000..d465f90ae
--- /dev/null
+++ b/tests/Unit/Messaging/MulticastSendReportTest.php
@@ -0,0 +1,40 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Kreait\Firebase\Tests\Unit\Messaging;
+
+use Iterator;
+use Kreait\Firebase\Exception\Messaging\NotFound;
+use Kreait\Firebase\Exception\Messaging\SenderIdMismatch;
+use Kreait\Firebase\Exception\MessagingException;
+use Kreait\Firebase\Messaging\MessageTarget;
+use Kreait\Firebase\Messaging\MulticastSendReport;
+use Kreait\Firebase\Messaging\SendReport;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @internal
+ */
+final class MulticastSendReportTest extends TestCase
+{
+    #[DataProvider('unknownTokenErrors')]
+    public function testItReturnsUnknownTokens(MessagingException $error): void
+    {
+        $target = MessageTarget::with(MessageTarget::TOKEN, 'token-from-another-project');
+        $sendReport = SendReport::failure($target, $error);
+        $report = MulticastSendReport::withItems([$sendReport]);
+
+        $this->assertSame(['token-from-another-project'], $report->unknownTokens());
+    }
+
+    /**
+     * @return Iterator<string, array{MessagingException}>
+     */
+    public static function unknownTokenErrors(): Iterator
+    {
+        yield 'not found' => [new NotFound('Not found')];
+        yield 'sender ID mismatch' => [new SenderIdMismatch('SenderId mismatch')];
+    }
+}