Lille appeared to be heading to a slender victory, but Remi Oudin found the net in the 78th minute and the goal was allowed to stand despite VAR images suggesting the winger was marginally offside, meaning PSG can wrap up the title at home to Strasbourg later on Sunday.
Although Christophe Galtier's men generally controlled proceedings before half-time, there were few incidents to excite either set of supporters.
Jose Fonte eventually put Lille ahead in the second half, profiting from woeful marking at a corner, but Europa League-hopefuls Reims pegged them back late on, doing PSG a favor.
Lille spent most of the first half in the ascendancy, but the only genuine opportunity they crafted arrived in the 16th minute, as Loic Remy shot straight at Edouard Mendy after being found by Nicolas Pepe.
But they finally made the breakthrough with 55 minutes played, as Fonte rose highest and sent a towering header into the top-right corner from Thiago Mendes' set-piece delivery.
The visitors had two narrow escapes at the other end just past the hour – Yunis Abdelhamid and Bjorn Engels forcing a reflex save from Mike Maignan coming roughly a minute apart.
They were not so fortunate 12 minutes from the end, though, as Oudin tucked under Maignan and VAR officials determined the goal to be legal despite there being a suggestion of offside.
There was more controversy to come as Lille defender Adama Soumaoro kicked the ball against his own hand in the box, but no review took place and Reims have prevented the chance to take all three points in stoppage time.