Ligue 1 Wrap: PSG fail to capitalise on slip-ups
Those blips earlier in the weekend had opened the door for the champions to reclaim top spot.
But Laurent Blanc's men wasted a host of opportunities in front of goal and, as a result, stay in second, a point behind Lyon.
PSG and Monaco meet again in the Coupe de France quarter-finals on Wednesday.
Monaco moved above Saint-Etienne and into fourth, two points ahead of Montpellier, who beat Nice 2-1 at home on Sunday.
Rolland Courbis' side had to come from behind to do so, though, after the visitors took a 36th-minute lead from the penalty spot.
Bryan Dabo levelled with a powerful effort at the back post a minute before half-time and Lucas Barrios added the winner in the 66th minute.
Yacine Bammou scored the only goal in the day's other game as Nantes climbed to eighth by inflicting a third straight defeat in all competitions on Guingamp, who face Concarneau in the Coupe de France on Thursday.