diff --git a/tests/handlers/test_cas.py b/tests/handlers/test_cas.py
index 8582b1cd1e9e..167c07d172a5 100644
--- a/tests/handlers/test_cas.py
+++ b/tests/handlers/test_cas.py
@@ -197,6 +197,29 @@ def test_required_attributes(self) -> None:
             auth_provider_session_id=None,
         )
 
+    @override_config(
+        {
+            "cas_config": {
+                "enable_registration": False
+            }
+        }
+    )
+    def test_map_cas_user_does_not_register_new_user(self) -> None:
+        """Ensures new users are not registered if the enabled registration flag is disabled."""
+        
+        auth_handler = self.hs.get_auth_handler()
+        auth_handler.complete_sso_login = AsyncMock()  # type: ignore[method-assign]
+        cas_response = CasResponse("test_user", {})
+        request = _mock_request()
+        self.get_success(
+            self.handler._handle_cas_response(request, cas_response, "redirect_uri", "")
+        )
+        self.complete_sso_login.assert_not_called()
+        self.assertRenderedError(
+            "mapping_error",
+            "User does not exist and registrations are disabled",
+        )
+
 
 def _mock_request() -> Mock:
     """Returns a mock which will stand in as a SynapseRequest"""