PSG, who have spent over 80 million euros on transfers for this season, prevailed with goals by Nene and Javier Pastore to move on to 70 points with three games left.
Montpellier, with Ligue 1's 14th highest budget, are on 73 after a 2-2 draw against Evian Thonon Gaillard on Tuesday.
"We were solid. Everybody thought we would finish third after last Sunday's game," PSG defender Zoumana Camara told Orange Sport.
"Now everybody can see we're still in the title race. We will focus on ourselves. We are still here, we will fight until the end."
Lille, who will travel to Montpellier in the penultimate round of matches, still have an outside chance of retaining their title after Tulio de Melo's early goal earned them a 1-0 win at strugglers Nice.
Rudi Garcia's side have 68 points after securing their fourth win in a row.
"This result puts us in good position," Garcia told reporters.
"If we want to leapfrog the teams in front of us, we will need to continue like this and hope for them to falter."
Carlo Ancelotti's PSG went ahead on 20 minutes when they were awarded a controversial penalty when Jeremy Menez appeared to have been brought down after missing the target with an angled shot.
Nene converted the penalty to give the home side a deserved lead that Menez failed to double when his low shot from just inside the box struck the post.
Stephane Ruffier punched away Mathieu Bodmer's volley on the stroke of half-time as Saint-Etienne struggled to contain PSG.
Pastore put the result beyond doubt when he tapped in from a Christophe Jallet cross two minutes from time.
Olympique Lyon, who have a game in hand, are fourth on 59 points after a Lisandro Lopez double and goals by Cris and Jimmy Briand gave them a 4-1 win against visitors Valenciennes.
Stade Rennes slipped to fifth on 57 points after losing 2-0 at Girondins Bordeaux.
Sochaux are second from bottom, one point from safety, after suffering a 2-1 defeat at AC Ajaccio and finishing the game with nine men.
