Lorient and AJ Auxerre drew 0-0 to move up a place to fifth and fourth on 26 and 27 points respectively after 16 games.
Marseille's Argentine defender Gabriel Heinze opened the scoring three minutes after the break at the Stade Velodrome with a header from Benoit Cheyrou's free kick.
"It was not easy against a side playing very defensively," Marseille coach Didier Deschamps told reporters. "But we don't get to be choosy. It's three points in the bag."
Valenciennes slipped to seventh after losing 2-1 at strugglers Le Mans. Anthony Le Tallec hit a last-gasp winner for Le Mans who were reduced to 10 men after Modibo Maiga's sending off in first-half stoppage time.
Racing Lens' Kanga Akale scored one and set up another to help his side move further away from the relegation zone with a 2-0 home victory against Nice.
