Yacine Brahimi scored the winner in the 19th minute to take Rennes to 37 points, four points behind leaders Lille, and they now trail second-placed PSG only on goal difference.
Champions Olympique Marseille were booed off the pitch after an unimpressive 1-0 win over bottom club Arles-Avignon.
But the result kept their title hopes alive as they climbed to fourth place on 36 points, one ahead of Saint Etienne who are fifth after a 2-1 win at Montpellier.
Rennes's Brahimi struck with a low shot from 20 metres against PSG who created several chances in the dying minutes but could not find a way past goalkeeper Nicolas Douchez.
"We got the chances to come back but we didn't manage anything. It's a shame to lose like that," PSG midfielder Clement Chantome told French TV channel Orange sport.
"There are a lot of matches left so it is not that serious but we will need to react," he added.
Rennes have themselves reacted convincingly after two demoralising setbacks in the past week - first losing 5-1 to Sochaux and then suffering a humiliating 4-3 elimination to Ligue 2 side Stade Reims in the last 16 of the French Cup.
"We received a lot of criticism these last few days. We were supposedly a club in crisis and we answered in the best possible way", Douchez said.
NOT BEAUTIFUL
Marseille's Andre-Pierre Gignac scored the winner from close-range at the Stade Velodrome after a perfectly-timed cross from fellow striker Brandao in the 55th minute.
Criticised for his poor form since joining Marseille last summer, the France player showed he was back to form and his performance was greeted with cheers rather than the usual jeers.
The team's overall performance, though, did nothing to ease the home crowd's worries after last week's disappointing draw at struggling Monaco.
"We did not play beautiful football. But the most important thing was the three points. We suffered a lot," defender Souleymane Diawara told French TV channel Foot Plus. "We need to give more if we want to achieve something."
Saint Etienne beat Montpellier thanks to a first-half double from 20-year-old Emmanuel Riviere. Jonas Martin's late goal was not enough to prevent the home side slipping to seventh.
Racing Lens stayed in the drop zone after a 1-1 draw with visiting neighbours Valenciennes while Caen are just outside the relegation places in 17th after a 2-0 defeat at home to Lorient.
