PSG missed the opportunity to climb to the top of Ligue 1 as goals from Paul Lasne, Ellyes Skhiri and Ryad Boudebouz condemned them to a surprise 3-0 defeat at struggling Montpellier.
The hosts went into the game just two points above the relegation zone but took the lead through Lasne's neat chip just before the break, then doubled their advantage early in the second half courtesy of a stunning curler from Skhiri.
As PSG laboured, they were undone again by Boudebouz's late solo effort in an uncharacteristically one-sided affair.
Angel Di Maria, Christopher Nkunku and Lucas Moura all went close for the champions, but despite dominating for long periods, they looked largely toothless in front of goal.
Defeat was PSG's third of the season and their first in the league against Montpellier since 2010.
Frederic Hantz's men, meanwhile, produced a sparkling performance - responding to recent criticism from their coach - as they improved upon a poor recent record of just one win from six league games.
They were grateful, however, to goalkeeper Laurent Pionnier, who made several fine saves to keep Nkunku, Di Maria and Lucas at bay.
Although dominant in terms of possession, PSG only managed to unpick Montpellier's obdurate, five-man defence once in the first half.
Di Maria provided the cross from the right, but Pionnier was equal to Nkunku's header.
Di Maria then skied a long-range effort over the crossbar and Lucas scuffed a shot wide from the edge of the box on the half-hour mark.
The hosts, meanwhile, took the lead from the only chance they managed to create before the break.
Hilton's long ball was allowed to reach Lasne just inside the box and as Alphonse Areola rushed unnecessarily off his line, the wing-back calmly lobbed him to claim his first goal of the season.
Montpellier doubled their lead in spectacular style three minutes into the second half.
Morgan Sanson showed great persistence to dispossess Marquinhos and then cut the ball cleverly back to the edge of the box, from where the onrushing Skhiri curled a superb shot into the right-hand corner of Areola's net.
PSG mounted an immediate response, but Pionnier did superbly to block a fierce strike from Di Maria and followed that up with another fine save to foil Lucas Moura.
Blaise Matuidi then squandered two decent chances and substitute Hatem Ben Arfa rifled a stinging drive just over the crossbar, as Unai Emery's men chased the game late on.
The final say went to the home side as Boudebouz capped a surging run that began midway inside his own half by crashing a 20-yard shot past Areola with 10 minutes left.