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.
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.
"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.
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.
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.
