Lucas Ocampos cancelled out Ola Toivonen's first-half opener but it was not enough as Marseille failed to win away from home in Ligue 1 for the seventh time in a row.
Marseille, who last clinched an away win on Oct.4, are second with 48 points from 24 matches, one point adrift of leaders Olympique Lyonnais.
Lyon take on third-placed Paris St Germain, who have 47 points, at Gerland on Sunday (2000 GMT).
In Rennes, Vincent Pajot headed the ball into the path of the unmarked Toivonen and the Sweden striker fired home with a low shot from inside the box in the 27th minute.
Marseille were lucky not to give away a penalty three minutes before the break as Giannelli Imubla appeared to handle the ball in the box, but referee Amaury Delerue did not blow.
The visitors' poor first half prompted coach Marcelo Bielsa to substitute winger Romain Alessandrini for Ocampos, who made his first OM appearance since he was taken on loan from Monaco.
Argentine Ocampos had an immediate impact, volleying home Dimitri Payet's corner kick on the hour.
Marseille finished the game with 10 men after Mario Lemina was shown a straight red card for kicking Toivonen seven minutes from time.
(Reporting by Julien Pretot; Editing by Mark Meadows)
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