PSG wrapped up the league title over 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 their campaign even before they arrived at a raucous Stade Auguste-Delaune.
Reims were good value for their 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 their 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 were 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 champions could not build any momentum and were 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 Rahman's long ball and calmly slotted past a stranded Buffon.
What does it mean? Dismal end to disappointing season
PSG have made a habit of dominating in France while faltering in Europe, but even their domestic form tailed off in 2018-19. The champions looked to have stopped the rot with back-to-back wins against Angers and Dijon, yet they were completely outplayed on Friday and questions will again be asked of Thomas Tuchel.
Loan star Rahman is still on Chelsea's books and he showed flashes of the talent that earned him a move to Stamford Bridge in 2015. The defending and goalkeeping left a lot to be desired, but his energy in bursting clear to score the opener typified a relentless Reims display. A late assist was an unexpected bonus.
Buffon, now 41 and preferred to Alphonse Areola, ended the season with a poor showing. Having already looked unconvincing from a couple of shots from range, he was caught out by low efforts for both goals and was all at sea when Chavarria finished PSG off.
Key Opta Facts
- Reims clinched their biggest win against PSG in Ligue 1 since July 1978 (2-0).
- PSG picked up 91 points in Ligue 1 this season – the fourth best tally for a team in a single campaign (three pts for a win) after PSG in 2015-16 (96), Monaco in 2016-17 (95) and Paris in 2017-18 (93).
- PSG are the first team to score in 100 per cent of their games in a Ligue 1 campaign with 20 teams.
- No player has scored more goals than Kylian Mbappe in Ligue 1 since 2017-18 (46 goals). With 33 goals in Ligue 1 this season, he set the best tally for a French player in a single season since Philippe Gondet in 1965-66 (36).
Another huge close season awaits PSG. Tuchel may well stay for a second season, but speculation surrounding Mbappe – who hinted at a potential exit after being named Ligue 1's Best Player and Best Young Player of the Year on Sunday – and Neymar will surely persist. And given the coach's repeated complaints about his squad size, players need to arrive, too.