PSG was left stunned at the Stade Auguste Delaune II on Saturday as it fell to 10-man Reims in a 1-0 loss in Ligue 1 action on Saturday.
Carlo Ancelotti's side remains top of the Ligue 1 table on 54 points, but the loss opens the door for Lyon to go level at the summit with a win over Brest on Sunday.
PSG thought it had claimed the lead just after the break when Zlatan Ibrahimovic broke through, but the goal was disallowed with the Swede being ruled offside.
The match turned in favor of PSG at the hour mark when Christopher Glombard was shown a second yellow card for bringing down Javier Pastore.
But despite playing down a man, it was the home side that took the lead as Grzegorz Krychowiak made the most of a corner kick in the 64th minute to hand Reims the win.
Reims improves to 27 points to pull further away from the relegation zone.
St. Etienne made up some ground on PSG on Saturday by cruising to a 4-0 win over 10-man Nice at the Stade Geoffroy-Guichard. Pierre-Emerick Aubameyang opened the scoring in the seventh minute from the penalty spot before the home side held a man advantage. Valentin Eysseric was sent off for Nice in the 24th minute, allowing St. Etienne to collect all three points behind a Brandao brace and Yohan Mollo.
Elsewhere in Ligue 1 on Saturday, Gael Danic's late penalty saw Valenciennes earn a point from a 1-1 draw with Lorient, Nancy picked up a 2-1 win at Sochaux, Bastia emerged as 1-0 winners against Ajaccio in a match that saw four red cards, and Toulouse played to 0-0 draw with Evian.