Marseille have 18 points and still enjoy a three-point lead over second-placed Paris St Germain. Valenciennes climbed to sixth on 11 points.
Marseille had the best defence before kick-off with only one goal conceded in their six previous league matches but were sunk at Valenciennes's Stade du Hainaut in a torrid first half.
Gael Danic curled a free kick into the near top corner in the 16th minute, the first time Marseille had fallen behind for the first time of the season, and conceded another when Danic burst into the box and set up Anthony Le Tallec on 34 minutes.
Goalkeeper Steve Mandanda then made a complete mess of an underarm throw five minutes later and the ball landed on Foued Kadir, who fired home to make it 3-0.
Le Tallec wrapped it up after the hour mark with a downward header before being replaced to a standing ovation five minutes from time. Marseille's Jordan Ayew scored a consolation goal in added time with a fine volley.