Paris St-Germain lost at home in the league for the first time since May 2018 as they were beaten 2-0 by Reims.
The French champions beat Real Madrid 3-0 in the Champions League last week but looked a shadow of that side in this game, failing to truly test Reims goalkeeper Predrag Rajkovic.
Instead, Reims took the lead in the first half when Hassane Kamara headed home.
Boulaye Dia’s bicycle kick in injury time gave Reims a deserved win.
PSG remain top of Ligue 1 with 15 points from seven games, with Reims eighth on 11.