diff --git a/src/google/adk/auth/oauth2_discovery.py b/src/google/adk/auth/oauth2_discovery.py
index ef509102a1..e9220e23ea 100644
--- a/src/google/adk/auth/oauth2_discovery.py
+++ b/src/google/adk/auth/oauth2_discovery.py
@@ -89,7 +89,7 @@ async def discover_auth_server_metadata(
           response.raise_for_status()
           metadata = AuthorizationServerMetadata.model_validate(response.json())
           # Validate issuer to defend against MIX-UP attacks
-          if metadata.issuer == issuer_url.rstrip("/"):
+          if metadata.issuer.rstrip("/") == issuer_url.rstrip("/"):
             return metadata
           else:
             logger.warning(
diff --git a/tests/unittests/auth/test_oauth2_discovery.py b/tests/unittests/auth/test_oauth2_discovery.py
index 48d755337d..88a87cd8c3 100644
--- a/tests/unittests/auth/test_oauth2_discovery.py
+++ b/tests/unittests/auth/test_oauth2_discovery.py
@@ -208,6 +208,29 @@ async def test_discover_auth_server_metadata_discard_mismatched_issuer(
         ),
     ])
 
+  @patch("httpx.AsyncClient.get")
+  @pytest.mark.asyncio
+  async def test_discover_auth_server_metadata_issuer_trailing_slash(
+      self,
+      mock_get,
+      auth_server_metadata,
+  ):
+    """Test issuer match tolerates a trailing slash on the returned issuer.
+
+    Some servers (e.g. FastMCP with GoogleProvider) return an issuer with a
+    trailing slash even though the requested issuer_url has none.
+    """
+
+    auth_server_metadata.issuer = "https://auth.example.com/"
+    mock_get.side_effect = [
+        self.mock_success_response(auth_server_metadata),
+    ]
+    discovery_manager = OAuth2DiscoveryManager()
+    result = await discovery_manager.discover_auth_server_metadata(
+        "https://auth.example.com"
+    )
+    assert result == auth_server_metadata
+
   @patch("httpx.AsyncClient.get")
   @pytest.mark.asyncio
   async def test_discover_resource_metadata_failed(