Late Niang winner fires Rennes back into third
Senegal international Niang helped Julien Stephan's side recover from a goal down and end a three-match winless streak in the league, while snapping lowly Nimes' four-match winning run.
Rennes are now one point clear of last season's runners-up Lille, who beat rock-bottom Toulouse 3-0 on Saturday, in the race for the third and final Champions League spot.
Marseille sit second, eight points ahead of Rennes but 10 points adrift of runaway leaders Paris Saint-Germain, who host Bordeaux later on Sunday
Nimes, whose strong recent run has given them fresh hope of avoiding relegation, grabbed the lead inside the first minute when Nolan Roux picked out the top corner.
Niang levelled just six minutes later, diverting in a loose ball inside the box.
Nimes centre-back Anthony Briancon was sent off early in the second half to leave the visitors down to 10 men, and Rennes eventually took advantage.
Niang was left unmarked to nod home his 10th league goal of the campaign.