diff --git a/src/Auth.php b/src/Auth.php
index fecc25b..3a86e66 100644
--- a/src/Auth.php
+++ b/src/Auth.php
@@ -9,6 +9,7 @@ class Auth
     private string $authcid;
     private string $secret;
     private string $authzid;
+    private string $authSource;
     private string $gs2Header;
     private ?string $cnonce = null;
     private string $firstMessageBare;
@@ -23,6 +24,7 @@ public function __construct(array $options)
         $this->authcid = $options['authcid'] ?? '';
         $this->secret = $options['secret'] ?? '';
         $this->authzid = $options['authzid'] ?? '';
+        $this->authSource = $options['authSource'] ?? 'admin';
         $this->cnonce = base64_encode(random_bytes(32));
     }
 
@@ -39,7 +41,7 @@ public function start(): array
                 "autoAuthorize" => 1,
                 "options" => ["skipEmptyExchange" => true],
             ],
-            'admin'
+            $this->authSource
         ];
     }
 
@@ -61,7 +63,7 @@ public function continue($data): array
                 "conversationId" => $cid,
                 "payload" => $payload,
             ],
-            'admin'
+            $this->authSource
         ];
     }
 
diff --git a/src/Client.php b/src/Client.php
index d54e9a6..f3f05e4 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -134,6 +134,11 @@ class Client
      * @param string $user
      * @param string $password
      * @param Boolean $useCoroutine
+     * @param bool $tls
+     * @param array $tlsOptions
+     * @param string|null $authSource Database to authenticate against; defaults to 'admin'.
+     *     Set this when the user was created in a database other than admin (e.g. the
+     *     application database itself).
      * @throws \Exception
      */
     public function __construct(
@@ -144,7 +149,8 @@ public function __construct(
         string $password,
         bool $useCoroutine = false,
         bool $tls = false,
-        array $tlsOptions = []
+        array $tlsOptions = [],
+        ?string $authSource = null
     ) {
         if (empty($database)) {
             throw new \InvalidArgumentException('Database name cannot be empty');
@@ -210,7 +216,8 @@ public function __construct(
 
         $this->auth = new Auth([
             'authcid' => $user,
-            'secret' => Auth::encodeCredentials($user, $password)
+            'secret' => Auth::encodeCredentials($user, $password),
+            'authSource' => $authSource ?? 'admin',
         ]);
     }
 
diff --git a/tests/AuthTest.php b/tests/AuthTest.php
new file mode 100644
index 0000000..1a95af2
--- /dev/null
+++ b/tests/AuthTest.php
@@ -0,0 +1,56 @@
+<?php
+
+namespace Utopia\Tests;
+
+use PHPUnit\Framework\TestCase;
+use Utopia\Mongo\Auth;
+
+class AuthTest extends TestCase
+{
+    public function testStartDefaultsToAdminAuthSource(): void
+    {
+        $auth = new Auth([
+            'authcid' => 'root',
+            'secret' => Auth::encodeCredentials('root', 'example'),
+        ]);
+
+        [$payload, $database] = $auth->start();
+
+        $this->assertSame(1, $payload['saslStart']);
+        $this->assertSame('admin', $database);
+    }
+
+    public function testStartUsesProvidedAuthSource(): void
+    {
+        $auth = new Auth([
+            'authcid' => 'appuser',
+            'secret' => Auth::encodeCredentials('appuser', 'example'),
+            'authSource' => 'appdb',
+        ]);
+
+        [$payload, $database] = $auth->start();
+
+        $this->assertSame(1, $payload['saslStart']);
+        $this->assertSame('appdb', $database);
+    }
+
+    public function testContinueUsesProvidedAuthSource(): void
+    {
+        $auth = new Auth([
+            'authcid' => 'appuser',
+            'secret' => Auth::encodeCredentials('appuser', 'example'),
+            'authSource' => 'appdb',
+        ]);
+
+        $auth->start();
+
+        $data = new \stdClass();
+        $data->conversationId = 1;
+        $data->payload = new \MongoDB\BSON\Binary('r=' . $auth->getCnonce() . 'server,s=' . base64_encode('salt') . ',i=4096', 0);
+
+        [$payload, $database] = $auth->continue($data);
+
+        $this->assertSame(1, $payload['saslContinue']);
+        $this->assertSame('appdb', $database);
+    }
+}