diff --git a/langchain/memory/chat_message_histories/mongodb.py b/langchain/memory/chat_message_histories/mongodb.py
index 3455d812..a6965af3 100644
--- a/langchain/memory/chat_message_histories/mongodb.py
+++ b/langchain/memory/chat_message_histories/mongodb.py
@@ -47,6 +47,7 @@ class MongoDBChatMessageHistory(BaseChatMessageHistory):
 
         self.db = self.client[database_name]
         self.collection = self.db[collection_name]
+        self.collection.create_index("SessionId")
 
     @property
     def messages(self) -> List[BaseMessage]:  # type: ignore