"When we are motivated like tonight or last weekend, we can definitely score goals," striker Guillaume Hoarau, who netted a double last Sunday when PSG snatched a 4-4 draw at Olympique Lyon, told Foot Plus TV.
"We wanted three points, we have them so we're happy. We wanted to regain our lead."
Argentine playmaker Javier Pastore, who joined from Palermo for a French record fee of 42 million euros ($55.5 million) in August, put the hosts in front when he fired past Guillermo Ochoa after a fine one-two with Blaise Matuidi on 27 minutes.
France midfielder Jeremy Menez doubled the tally two minutes later, curling a superb shot into the top corner.
Yohann Poulard pulled one back for the visitors, netting from close range three minutes before the break.
PSG, who are hoping to win their first French league title since 1994, wrapped up victory with four minutes left when Hoarau found the back of the net with a delicate chip.
Nene added to Ajaccio's relegation worries three minutes into stoppage time with a fourth.
Jonathan Pitroipa gave Rennes the lead when he converted a quick counter-attack four minutes before the break. The Burkina Faso winger then set up Youssouf Hadji, who netted a fine shot from outside the box to secure the win on 79 minutes.