The 10-time champions' third home league loss in a row increases the pressure on coach Laurent Roussey a few days after the club's joint presidents gave him two games to save his job.
"His fate is still undecided, we will listen to him tomorrow," Bernard Caiazzo, one of the two presidents, told reporters.
Caiazzo added that former Tottenham sporting director Damien Comolli had been named technical director of the club.
Stade Rennes took the lead in the 13th minute when Mickael Pagis (pictured) easily ran through the St Etienne defence to collect a Jimmy Briand pass and beat goalkeeper Jody Viviani from close range.
The away side sealed the victory just after the interval with two goals in as many minutes by defender Rod Fanni and Pagis.
St Etienne, who made it two UEFA Cup group stage wins out of two with a 3-0 victory over Rosenborg Trondheim on Thursday, are 18th with 10 points from 13 games.
Valenciennes remain bottom on nine points after a 1-0 home defeat to AS Nancy.
Olympique Lyon, who won 1-0 at Monaco on Saturday, lead the standings on 30 points, four ahead of Olympique Marseille, 3-0 winners at Grenoble.
Later on Sunday, Ludovic Giuly scored his first goal for Paris St Germain to help the capital side recover from two consecutive defeats with a 1-0 home win over Lille.
PSG are eighth on 20 points, the same as Lille.
