diff --git a/src/google/adk/agents/readonly_context.py b/src/google/adk/agents/readonly_context.py
index 548425367..3abf81ae9 100644
--- a/src/google/adk/agents/readonly_context.py
+++ b/src/google/adk/agents/readonly_context.py
@@ -33,6 +33,11 @@ def __init__(
   ) -> None:
     self._invocation_context = invocation_context
 
+  @property
+  def user_id(self) -> str:
+    """The user id that start this invocation. READONLY field."""
+    return self._invocation_context.user_id
+
   @property
   def user_content(self) -> Optional[types.Content]:
     """The user content that started this invocation. READONLY field."""
diff --git a/tests/unittests/agents/test_readonly_context.py b/tests/unittests/agents/test_readonly_context.py
index c2ffa6e0a..0e4f52e40 100644
--- a/tests/unittests/agents/test_readonly_context.py
+++ b/tests/unittests/agents/test_readonly_context.py
@@ -8,6 +8,8 @@
 @pytest.fixture
 def mock_invocation_context():
   mock_context = MagicMock()
+
+  mock_context.user_id = "test-user-id"
   mock_context.invocation_id = "test-invocation-id"
   mock_context.agent.name = "test-agent-name"
   mock_context.session.state = {"key1": "value1", "key2": "value2"}
@@ -15,6 +17,12 @@ def mock_invocation_context():
   return mock_context
 
 
+def test_user_id(mock_invocation_context):
+  readonly_context = ReadonlyContext(mock_invocation_context)
+  print(readonly_context.__dict__)
+  assert readonly_context.user_id == "test-user-id"
+
+
 def test_invocation_id(mock_invocation_context):
   readonly_context = ReadonlyContext(mock_invocation_context)
   assert readonly_context.invocation_id == "test-invocation-id"