diff --git a/python_tests/test_api.py b/python_tests/test_api.py
index ce78da47..8aa8fac2 100644
--- a/python_tests/test_api.py
+++ b/python_tests/test_api.py
@@ -220,6 +220,49 @@ class APITestCase(TestCase):
         assert study_detail["feedback_component_type"]["output_type"] == "artifact"
         assert study_detail["feedback_component_type"]["artifact_key"] == "image"
 
+    def test_save_trial_user_attrs(self) -> None:
+        study = optuna.create_study()
+        trials: list[optuna.Trial] = []
+        for _ in range(2):
+            trial = study.ask()
+            trials.append(trial)
+
+        request_body = {
+            "user_attrs": {
+                "number": 0,
+            },
+        }
+
+        app = create_app(study._storage)
+        status, _, _ = send_request(
+            app,
+            f"/api/trials/{trials[0]._trial_id}/user-attrs",
+            "POST",
+            content_type="application/json",
+            body=json.dumps(request_body),
+        )
+        self.assertEqual(status, 204)
+
+        assert study.trials[0].user_attrs == request_body["user_attrs"]
+        assert study.trials[1].user_attrs == {}
+
+
+    def test_save_trial_user_attrs_empty(self) -> None:
+        study = optuna.create_study()
+        trial = study.ask()
+
+        app = create_app(study._storage)
+        status, _, _ = send_request(
+            app,
+            f"/api/trials/{trial._trial_id}/user-attrs",
+            "POST",
+            content_type="application/json",
+            body=json.dumps({}),
+        )
+        self.assertEqual(status, 400)
+        assert study.trials[0].user_attrs == {}
+
+
     @pytest.mark.skipif(sys.version_info < (3, 8), reason="BoTorch dropped Python3.7 support")
     def test_skip_trial(self) -> None:
         storage = optuna.storages.InMemoryStorage()