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.
Stephane Ruffier punched away Mathieu Bodmer's volley on the stroke of half-time as Saint-Etienne struggled to contain PSG.
