Ligue 1 Review: PSG stutter, Saint-Etienne respond
Paul-Georges Ntep's bizarre goal denied Laurent Blanc's champions, who have drawn three of their first five games this term.
PSG thought they were on course for three points at the Stade de Route de Lorient as Zoumana Camara made the most a rare start to open the scoring shortly before half-time.
However, Rennes clinched a share of the spoils when Ntep, who had only just come on as a substitute, was credited with the equaliser when it was deemed that his low cross had not touched Abdoulaye Doucoure on its way into the back of the net.
Unbeaten PSG failed to reach the heights of their 5-0 victory against Saint-Etienne before the international break, although Zlatan Ibrahimovic - who scored a hat-trick in that fixture - did rattle the crossbar.
Christophe Galtier's side provided a fine response to their capital thrashing, edging Caen 1-0.
Jean-Jacques Pierre turned substitute Yohan Mollo's cross into his own net with Saint-Etienne striker Mevlut Erding awaiting a tap-in and that proved to be enough for all three points.
The win takes Saint-Etienne joint top with Bordeaux, who face Guingamp on Sunday.
David N'Gog grabbed a debut goal for Reims as they overcame Toulouse 2-0 for their first win of the season, Gaetan Courtet made sure of the points four minutes from time.
Lamine Kone's own goal handed Montpellier a 1-0 win over Lorient to keep the 2012 champions in touch at the top and Nice beat Metz by the same scoreline thanks to a 95th-minute goal from Alexy Bosetti.
Elsewhere, Bastia and Lens drew 1-1 at the Stade Armand Cesari.