PSG wrapped up the league title more than a month ago, but a run of three wins in nine in all competitions — a stretch that included a Coupe de France final loss to Rennes — had put a slight dampener on its campaign even before it arrived at a raucous Stade Auguste-Delaune.
Reims was good value for its two-goal lead with strikes from Baba Rahman and Mathieu Cafaro either side of half-time, although Kylian Mbappe brought the visitors back into contention in the 59th minute.
But PSG mustered little more without the suspended Neymar and substitute Pablo Chavarria scored an opportunistic third in second-half stoppage-time to compound its misery.
Reims seized control midway through the first half and superb work from Cafaro teed up Remi Oudin to thunder straight at Gianluigi Buffon, before a first-time Moussa Doumbia curler clipped the crossbar.
The opener deservedly followed in the 36th minute when lax defending from Thomas Meunier allowed Rahman in to slide a finish through Buffon's legs from a tight angle.
PSG was initially much improved after the break and Thilo Kehrer glanced a header wide, but Reims retained a threat on the break and doubled their lead when Cafaro drove a fierce strike past Buffon's flimsy right hand in the 56th minute.
Although Mbappe responded quickly by turning Edinson Cavani's header across the line at close range, the champion could not build any momentum and was picked off in the fourth minute of added time.
Chavarria, who had been sent on for Cafaro, was given the freedom of the PSG half after chasing down Baba's long ball and calmly slotted past a stranded Buffon.