Thomas Tuchel's side could have been crowned champions in record-equalling time last weekend but failed to beat Strasbourg, and they fell short again on Sunday despite needing just a point in a match that saw them have to play with 10 men for 54 minutes and lose both captain Thiago Silva and Thomas Meunier to injury.
A comical Meunier own goal got the hosts on their way after six minutes, but PSG looked set to seal the title when Juan Bernat levelled soon after and Kylian Mbappe roofed what looked like his 28th league goal of the season in the 13th minute, only to see it ruled out for offside.
Bernat was then handed a debatable straight red card in the 36th minute for a tug on Nicolas Pepe despite a VAR check, and after attacker Pepe slotted home Lille's second five minutes after the break the floodgates opened.
Jonathan Bamba smashed home the third 14 minutes later and two fine headers from Gabriel and Jose Fonte completed the rout, PSG's worst Ligue 1 loss since going down 4-0 at Bordeaux in 2009.
Despite conceding five goals in the league for the first time since another 5-1 defeat at Sedan way back in 2000, Thomas Tuchel's side are still 17 points clear of Sunday's opponents with a game in hand and odds-on to retain the title.