Disclosed is a system and a method for session reestablishment between a client terminal and a server for a messenger service. In a session reestablishment system including a client terminal capable of accessing a communication network, a session information database and a server for providing a messenger service to the client terminal, the client terminal sends a synckey exchanged and stored during previous access to the server when sending a request for session reestablishment (reaccess) to the server after lapse of a predetermined time period from termination of the previous access due to log-off or timeout. The server searches the session information database to detect last session information corresponding to the client terminal based on the synckey, compares the last session information with current session information to detect any updated data, and sends only the updated data to the client terminal.