Montpellier once again benefited from a sterling performance by Younes Belhanda, who struck a double, including a superb volley into the top corner as the visitors claimed their first win at Marseille from 21 attempts with Olivier Giroud also on the scoreboard.
After handing Marseille their 11th defeat in 12 outings in all competitions, Montpellier moved up to 66 points with seven games left. Second-placed Paris Saint-Germain's are on 63.
Marseille, who are hoping to win the League Cup final against Olympique Lyon on Saturday to save their season, stayed ninth on 40 points, eight points above the relegation zone.
Olympique Marseille's largest fans group, the South Winners, had asked their team to lose against Montpellier in order to prevent arch rivals PSG from clinching the title.
"Stay true to yourselves... lose", claimed a banner in the Stade Velodrome.
Marseille duly delivered as both teams played a lively game in hand.
Belhanda put Montpellier ahead in the seventh minute by converting a penalty which was awarded following a handball by Djimi Traore.
Marseille levelled on 33 minutes through Stephane Mbia's downward header from a Benoit Cheyrou free-kick.
Giroud scored his 17th goal of the season when he volleyed home a Souleymane Camara cross from the right four minutes into the second half.
Morocco midfielder Belhanda wrapped it up in the 71st minute when he chested down a Giroud cross and fired a swift volley into the top corner.
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