Kevin Gameiro scored a hat trick to take 1 on Sunday at the Stade Francois Coty to extend its unbeaten streak to nine matches.
PSG lost its first match of the season, but has seven wins and two ties in its last nine, and leads the standings with 23 points. Montpellier and Lyon sit on 20 points, level for second place.
Gameiro opened the scoring in the 2nd minute, but Ajaccio was on even terms in the 23rd when Carl Medjani scored his first of the season.
PSG was back in front five minutes after half as Gameiro - who found the lower left for his first goal - turned Mohamed Sissoko's pass into the center of the goal, then added his eighth goal of the season three minutes later.
Gameiro completed his hat trick with a shot to the bottom right, converting a pass from Clement Chantome past Mexican goalkeeper Guillermo Ochoa.
Ajaccio sits on just one win after 10 games and is just two points above last- place Nancy in the standings.
Valenciennes 3, Sochaux 0
Valenciennes, France - Vincent Aboubakar scored twice as Valenciennes defeated Sochaux, 3-0, on Sunday at the Stade Nungesser to end a three-game winless run in Ligue 1.
Aboubakar opened the scoring 2 minutes before haltime and, after Foued Kadir scored in the 71st, completed his double and sealed Valenciennes' victory with his second goal in the 75th.
Sochaux had just one loss in its first six games this season, but lost for the third time in its last four to fall 11 points off the pace of leaders PSG.
Rennes 2, Lorient 0
Rennes, France - Jires Ekoko scored his fourth goal of the season, and Rennes defeated Lorient, 2-0, on Sunday at the Stade de la Route de Lorient to remain just five points behind Ligue 1 leader PSG.
Ekoko scored in the 10th minute and Jonathan Pitroipa sealed Rennes' fifth win with a goal from the penalty spot early in the second half. Lorient lost for just the second time this season.