PSG dropped its first points of the season when it drew 2-2 at Bordeaux on Sunday.
Marseille twice took the lead before losing 3-2 at Nantes.
The visitors struck through midfielder Morgan Sanson and winger Florian Thauvin’s penalty, his 11th goal of the campaign, but striker Emiliano Sala replied with a firm header and then used his upper-body strength to help set up a goal for midfielder Abdoulaye Toure.
Sloppy Marseille defending then led to attacking midfielder Gabriel Boschilia’s close-range winner midway through the second half.
Sala is level at the top of the scoring charts on 12 goals with PSG’s Kylian Mbappe, who was on the bench for PSG’s game.