Despite racing ahead through Javier Pastore's strike and a Zlatan Ibrahimovic penalty, PSG had to settle for a point as St Etienne's spirited response brought an Alex own-goal and a Francois Clerc equaliser.
St Etienne remain unbeaten this year but will be disappointed not to have claimed the three points that would have taken them level with Marseille in third.
Nice also missed the chance to pull level with OM as the surprise European hopefuls slipped to a 1-0 defeat at relegation-threatened Nancy thanks to Djamel Bakar's first-half strike.
Nancy remain well adrift of safety, five points behind fourth-bottom Brest, but they have four wins in their last eight league outings.
In the day's other game, Toulouse and Bordeaux played out a dour goalless draw in the derby de la Garonne.