diff --git a/python_tests/test_api.py b/python_tests/test_api.py
index bde7838a2..ce78da47a 100644
--- a/python_tests/test_api.py
+++ b/python_tests/test_api.py
@@ -399,6 +399,125 @@ def test_delete_study_not_found(self) -> None:
         )
         self.assertEqual(status, 404)
 
+    def test_tell_trial_complete(self) -> None:
+        storage = optuna.storages.InMemoryStorage()
+        study = optuna.create_study(storage=storage)
+        trial_id = study.ask()._trial_id
+
+        app = create_app(storage)
+        status, _, _ = send_request(
+            app,
+            f"/api/trials/{trial_id}/tell",
+            "POST",
+            body=json.dumps(
+                {
+                    "state": "Complete",
+                    "values": [0, 1, 2],
+                }
+            ),
+            content_type="application/json",
+        )
+        self.assertEqual(status, 204)
+        trial = storage.get_trial(trial_id)
+        assert trial.state == optuna.trial.TrialState.COMPLETE
+        assert trial.values == [0, 1, 2]
+
+    def test_tell_trial_fail(self) -> None:
+        storage = optuna.storages.InMemoryStorage()
+        study = optuna.create_study(storage=storage)
+        trial_id = study.ask()._trial_id
+
+        app = create_app(storage)
+        status, _, _ = send_request(
+            app,
+            f"/api/trials/{trial_id}/tell",
+            "POST",
+            body=json.dumps(
+                {
+                    "state": "Fail",
+                }
+            ),
+            content_type="application/json",
+        )
+        self.assertEqual(status, 204)
+        trial = storage.get_trial(trial_id)
+        assert trial.state == optuna.trial.TrialState.FAIL
+
+    def test_tell_trial_with_no_state(self) -> None:
+        storage = optuna.storages.InMemoryStorage()
+        study = optuna.create_study(storage=storage)
+        trial_id = study.ask()._trial_id
+
+        app = create_app(storage)
+        status, _, _ = send_request(
+            app,
+            f"/api/trials/{trial_id}/tell",
+            "POST",
+            body=json.dumps({}),
+            content_type="application/json",
+        )
+        self.assertEqual(status, 400)
+
+    def test_tell_trial_with_invalid_state(self) -> None:
+        storage = optuna.storages.InMemoryStorage()
+        study = optuna.create_study(storage=storage)
+        for state in ["Pruned", "Running", "Waiting", "Invalid"]:
+            trial_id = study.ask()._trial_id
+            app = create_app(storage)
+            with self.subTest(state=state):
+                status, _, _ = send_request(
+                    app,
+                    f"/api/trials/{trial_id}/tell",
+                    "POST",
+                    body=json.dumps(
+                        {
+                            "state": state,
+                        }
+                    ),
+                    content_type="application/json",
+                )
+                self.assertEqual(status, 400)
+
+    def test_tell_trial_with_no_values(self) -> None:
+        storage = optuna.storages.InMemoryStorage()
+        study = optuna.create_study(storage=storage)
+        trial_id = study.ask()._trial_id
+
+        app = create_app(storage)
+        status, _, _ = send_request(
+            app,
+            f"/api/trials/{trial_id}/tell",
+            "POST",
+            body=json.dumps(
+                {
+                    "state": "Complete",
+                }
+            ),
+            content_type="application/json",
+        )
+        self.assertEqual(status, 400)
+
+    def test_tell_trial_with_invalid_values(self) -> None:
+        storage = optuna.storages.InMemoryStorage()
+        study = optuna.create_study(storage=storage)
+        for values in [1.0, ["foo"]]:
+            trial_id = study.ask()._trial_id
+            app = create_app(storage)
+            with self.subTest(values=values):
+                status, _, _ = send_request(
+                    app,
+                    f"/api/trials/{trial_id}/tell",
+                    "POST",
+                    body=json.dumps(
+                        {
+                            "state": "Complete",
+                            "values": values,
+                        }
+                    ),
+                    content_type="application/json",
+                )
+                self.assertEqual(status, 400)
+
 
 class BottleRequestHookTestCase(TestCase):
     def test_ignore_trailing_slashes(self) -> None: