diff --git a/src/google/adk/memory/in_memory_memory_service.py b/src/google/adk/memory/in_memory_memory_service.py
index 129ba5bcb..8b6f36138 100644
--- a/src/google/adk/memory/in_memory_memory_service.py
+++ b/src/google/adk/memory/in_memory_memory_service.py
@@ -14,6 +14,7 @@
 from __future__ import annotations
 
 import re
+import threading
 from typing import TYPE_CHECKING
 
 from typing_extensions import override
@@ -42,38 +43,43 @@ class InMemoryMemoryService(BaseMemoryService):
 
   Uses keyword matching instead of semantic search.
 
-  It is not suitable for multi-threaded production environments. Use it for
-  testing and development only.
+  This class is thread-safe, however, it should be used for testing and
+  development only.
   """
 
   def __init__(self):
+    self._lock = threading.Lock()
+
     self._session_events: dict[str, dict[str, list[Event]]] = {}
-    """Keys are app_name/user_id, session_id. Values are session event lists."""
+    """Keys are "{app_name}/{user_id}". Values are dicts of session_id to
+    session event lists.
+    """
 
   @override
   async def add_session_to_memory(self, session: Session):
     user_key = _user_key(session.app_name, session.user_id)
-    self._session_events[user_key] = self._session_events.get(
-        _user_key(session.app_name, session.user_id), {}
-    )
-    self._session_events[user_key][session.id] = [
-        event
-        for event in session.events
-        if event.content and event.content.parts
-    ]
+
+    with self._lock:
+      self._session_events[user_key] = self._session_events.get(user_key, {})
+      self._session_events[user_key][session.id] = [
+          event
+          for event in session.events
+          if event.content and event.content.parts
+      ]
 
   @override
   async def search_memory(
       self, *, app_name: str, user_id: str, query: str
   ) -> SearchMemoryResponse:
     user_key = _user_key(app_name, user_id)
-    if user_key not in self._session_events:
-      return SearchMemoryResponse()
+
+    with self._lock:
+      session_event_lists = self._session_events.get(user_key, {})
 
     words_in_query = set(query.lower().split())
     response = SearchMemoryResponse()
 
-    for session_events in self._session_events[user_key].values():
+    for session_events in session_event_lists.values():
       for event in session_events:
         if not event.content or not event.content.parts:
           continue