Marseille have 18 points and still enjoy a three-point lead over second-placed Paris Saint-Germain, who beat Sochaux 2-0 on Saturday one week before the arch-rivals clash at the Stade Velodrome.
Olympique Lyon, on 14 points, slipped to third after visiting Girondins Bordeaux beat them 2-0 to hand the seven times French champions their first defeat of the season.
Bordeaux, who are still unbeaten, climbed to fifth on 13 points, two points ahead of seventh-placed Valenciennes.
Marseille had the best defensive record before the kickoff 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 top corner in the 16th minute, the first time Marseille had fallen behind this 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 by Foued Kadir, who fired home to make it 3-0.
Le Tallec wrapped the match 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.
"It's a heavy defeat. We did not start the game well and then we missed a few opportunities to level for 1-1," Marseille coach Elie Baup told TV channel BeIN Sport.
"When you concede such a defeat it means that a lot of things went wrong. However, we need to put this aside and focus on a new week."
Marseille host Limassol FC in the Europa League on Thursday before they play Paris Saint-Germain in Ligue 1 next Sunday.
"We have to look forward," said Baup. "We have two important games next week so let's analyse what went wrong."
Later on Sunday, Lyon dominated the first half but Clement Grenier and Maxime Gonalons were denied by Cedric Carrasso as Bordeaux held firm.
The visitors went ahead in the 65th minute when Benoit Tremoulinas played a fine one-two with Ludovic Obraniak and curled his shot past Remy Vercoutre.
Lyon piled the pressure but at the end of a counter-attack, Cheick Diabate benefited from some shaky defending by Dejan Lovren to head home and make it 2-0 eight minutes from time.
Elsewhere Saint-Etienne and Stade Reims drew 0-0.
