Marseille took an early lead against the run of play thanks to France forward Hatem Ben Arfa.
Argentina's Lucho Gonzalez and fellow midfielder Benoit Cheyrou secured Marseille's win after the break.
"This is tough for us as we got off to a good start and then we conceded a first goal which hurted us. 3-0 is a harsh score," PSG captain Claude Makelele told Canal Plus.
Marseille are on 48 points, three behind leaders Girondins Bordeaux whose visit to Le Mans, also scheduled for Sunday, was postponed due to high winds and heavy rains sweeping France. PSG are 12th on 33 points.
Montpellier moved level on points with Bordeaux thanks to a 3-1 home win over Stade Rennes on Saturday. Bordeaux, who have two games in hand, lead promoted Montpellier on goal difference on 51 points.
Lille dropped to fifth on 47 points with a 2-1 home defeat to Auxerre on Sunday while in-form Olympique Lyon are third on 49 points after a 2-0 win at home over Nice on Saturday.
CONSTANT PRESSURE
Marseille stood up to PSG's constant pressure in a feverish atmosphere at the Parc des Princes in first half and the Provence side opened the scoring in the 15th minute thanks to Ben Arfa.
The France forward fired home a low right foot shot from the edge of the box from a Laurent Bonnart free-kick.
Paris dominated throughout the first half and Marseille goalkeeper Steve Mandanda had to make two saves in succession before the interval to deny efforts by Guillaume Hoarau and Ludovic Giuly back.
Marseille came close to doubling their advantage in the 45th minute when a Gabriel Heinze header hit the post for the visitors' only second chance in the first half.
"During the break, I told my players that miracles did exist in soccer as we could have led 2-0," Marseille coach Didier Deschamps told Canal Plus.
"We let Paris create many chances in the first half but they loosened their grip after the break and we saw the difference between a side that is full of confidence and an other one who doubts."
Paris were far less aggressive after the break and Lucho Gonzalez silenced PSG fans when he calmly beat Apoula Edel from close range after the Paris goalkeeper had released the ball from a Heinze cross in the 54th minute.
Cheyrou sealed Marseille's win also from close range after being set up in the box by a Mamadou Niang header in the 71th minute.
A Roy Contout double condemned Lille to a 2-1 home defeat earlier on Sunday.
The French forward found the back of the net each side of the interval for the visitors. Lille had levelled at 1-1 in the first half thanks to Belgium striker Eden Hazard.
The game at Lille was delayed for two hours because of bad weather.
"We didn't have many chances but Roy was very efficient in front of goal. We deserve this victory," Auxerre coach Jean Fernandez told Foot Plus channel.
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