PSG's first home defeat in almost a year on Friday was a shock reverse for a club that clinched the French league title three weeks ago and was shifting its focus to the Champions League semifinal against Arsenal on Tuesday.
Coach Luis Enrique rested several starters but it was still one-way traffic as PSG dominated the early stages.
However, profligate finishing and an inspired performance by Nice goalkeeper Marcin Bulka kept PSG scoreless.
Nice's Morgan Sanson broke the deadlock when Badredine Bouanani's elegant through ball left him with only Gianluigi Donnarumma to beat and he slipped the ball neatly under the keeper.
The lead lasted only five minutes with PSG's Fabián Ruiz turning inside the box and firing home a neat chip over the defence from Ousmane Dembélé.
Nice did the unlikely again a minute into the second half and once more it was Sanson who did the damage, scoring with a spectacular scissor kick volley.
Even with the introduction of Bradley Barcola, Gonçalo Ramos, Warren Zaire-Emery, and Lucas Beraldo, the home side failed to react and, incredibly, Nice scored again.
An unmarked Youssouf Ndayishimiye headed home the third in the 70th minute.
The win lifted Nice into fourth place, equal on points with Monaco and a point behind second-placed Marseille.
PSG remain a healthy 22 points clear at the top of the table.
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