Paul Le Guen's PSG went ahead after 26 minutes when defender Mamadou Sakho headed home in a packed goalmouth but St Etienne hit back just seven minutes later.
Midfielder Paulo Machado's 25-metre drive smashed against Mickael Landreau's crossbar but striker Dimitri Payet followed up to poke the ball into the empty net.
Les Verts barely had time to celebrate before PSG midfielder Jeremy Clement, who played with a fractured hand, headed home Ludovic Giuly's cross eight minutes from the interval.
Girondins Bordeaux slipped one place to third after being held to a 1-1 home draw by promoted Grenoble leaving the Aquitaine side three points off top spot on 43.
Bordeaux opened the scoring on the hour when Morocco striker Marouane Chamakh headed home after Grenoble goalkeeper Gregory Wimbee failed to block Fernando Cavenaghi's attempt.
But Bostjan Cesar equalised 14 minutes from time when a Laurent Courtois shot bounced off Ulrich Rame's left post to the Slovenian defender's feet and he put it into the net.
Bordeaux started the game with the influential Yoann Gourcuff on the bench after the playmaker featured for all 90 minutes of France's 2-0 defeat by Argentina on Wednesday.
Gourcuff came on with nine minutes left but had no effect and it was Grenoble who had a chance to snatch all three points when Sofiane Seghouli's volley hit the bar after 84 minutes.
Lille, who are also on the fringes of the title race, slipped one place to seventh after losing 2-0 at AJ Auxerre and remain seven points behind the leaders.
Jimmy Briand's late goal helped Stade Rennes move up a spot to sixth after salvaging a 1-1 draw at home to Nancy, who had taken the lead courtesy of a Youssouf Hadji close-range shot.
