diff --git a/src/google/adk/agents/readonly_context.py b/src/google/adk/agents/readonly_context.py
index 425ab34712..6bd9183328 100644
--- a/src/google/adk/agents/readonly_context.py
+++ b/src/google/adk/agents/readonly_context.py
@@ -59,6 +59,11 @@ def state(self) -> MappingProxyType[str, Any]:
   def session(self) -> Session:
     """The current session for this invocation."""
     return self._invocation_context.session
+  
+  @property
+  def user_id(self) -> str:
+    """The id of the user. READONLY field."""
+    return self._invocation_context.user_id
 
   @property
   def run_config(self) -> Optional[RunConfig]:
diff --git a/tests/unittests/agents/test_readonly_context.py b/tests/unittests/agents/test_readonly_context.py
index fbea0b0aff..67a749d56e 100644
--- a/tests/unittests/agents/test_readonly_context.py
+++ b/tests/unittests/agents/test_readonly_context.py
@@ -11,6 +11,7 @@ def mock_invocation_context():
   mock_context.invocation_id = "test-invocation-id"
   mock_context.agent.name = "test-agent-name"
   mock_context.session.state = {"key1": "value1", "key2": "value2"}
+  mock_context.user_id = "test-user-id"
   return mock_context
 
 
@@ -31,3 +32,8 @@ def test_state_content(mock_invocation_context):
   assert isinstance(state, MappingProxyType)
   assert state["key1"] == "value1"
   assert state["key2"] == "value2"
+
+
+def test_user_id(mock_invocation_context):
+  readonly_context = ReadonlyContext(mock_invocation_context)
+  assert readonly_context.user_id == "test-user-id"