diff --git a/src/google/adk/sessions/vertex_ai_session_service.py b/src/google/adk/sessions/vertex_ai_session_service.py
index 8d6fa75..8387de6 100644
--- a/src/google/adk/sessions/vertex_ai_session_service.py
+++ b/src/google/adk/sessions/vertex_ai_session_service.py
@@ -57,6 +57,12 @@ class VertexAiSessionService(BaseSessionService):
       state: Optional[dict[str, Any]] = None,
       session_id: Optional[str] = None,
   ) -> Session:
+    if session_id:
+      raise ValueError(
+          'User-provided Session id is not supported for'
+          ' VertexAISessionService.'
+      )
+
     reasoning_engine_id = _parse_reasoning_engine_id(app_name)
 
     session_json_dict = {'user_id': user_id}
diff --git a/tests/unittests/sessions/test_vertex_ai_session_service.py b/tests/unittests/sessions/test_vertex_ai_session_service.py
index fe65731..d56bdf2 100644
--- a/tests/unittests/sessions/test_vertex_ai_session_service.py
+++ b/tests/unittests/sessions/test_vertex_ai_session_service.py
@@ -261,3 +261,15 @@ def test_create_session():
   assert session == session_service.get_session(
       app_name='123', user_id='user', session_id=session_id
   )
+
+
+def test_create_session_with_custom_session_id():
+  session_service = mock_vertex_ai_session_service()
+
+  with pytest.raises(ValueError) as excinfo:
+    session_service.create_session(
+        app_name='123', user_id='user', session_id='1'
+    )
+    assert str(excinfo.value) == (
+        'User-provided Session id is not supported for VertexAISessionService.'
+    )