Lille's game on the south coast was meandering to a goalless draw with neither side fashioning a clear-cut chance, when Younes Belhanda (pictured) struck a crisp low left-foot shot into the corner of the net on 84 minutes.
Modest Montpellier vaulted up to sixth, inside the Europa League qualifying spots, with a victory they just about deserved having had a first-half penalty appeal turned down.
The defeat, Lille's first in the league since October 24 and third this season, allowed second-placed Rennes to cut the gap to two points after a late penalty sealed a 2-1 victory at Toulouse.
Yann M'Vila's free-kick on half-time was too hot to handle for Toulouse goalkeeper Marc Vidal but the hosts levelled in bizarre fashion when Rennes keeper Nicolas Douchez punched the ball against team mate Kevin Theophile-Catherine and it flew in.
A bad night for keepers got worse when Vidal was carried off on a stretcher after a collision which prompted a penalty and Victor Montano converted on 88 minutes.
PSG moved to within four points of Lille in a congested top half of Ligue 1 after Ludovic Giuly, Guillaume Hoarau and Sylvain Armand sealed a 3-0 win at a woeful Nice, who are one place above the relegation zone.
Ex-Barcelona winger Giuly, 34, who is close to agreeing a contract extension, pounced following a corner after 24 minutes before striker Hoarau netted a classy second.
Centre-back Armand, poleaxed by his own goalkeeper in last weekend's disappointing draw with Racing Lens, recovered from head injuries to play and sealed the win midway through the second half following another dangerous Nene corner.
Champions Olympique Marseille and Olympique Lyon, title-holders from 2002-2008, had already put pressure on surprise packages Lille by beating Saint Etienne and AS Nancy respectively over the weekend.
