At a leadership conference Saturday, party members voted to oust former Prime Minister Laurent Fabius from his leadership post for advocating a "no" vote on the referendum. Most Socialists voted against the referendum.
The Socialist Party had voted in December to support the treaty.
Party chief Francois Hollande had warned Friday he would call for a unified group of party leaders, without specifically calling for Mr. Fabius' ouster.
Mr. Fabius is the second major French politician to lose his job in the fallout following the defeat of the referendum. French President Jacques Chirac reshuffled his government earlier this week, replacing Prime Minster Jean-Pierre Raffarin with former Interior Minister Dominique de Villepin.
Some information for this report provided by AFP and AP.