In a game that was put back 24 hours after storm warnings in the south of France on Sunday, Marseille took the lead after just five minutes and never looked like relinquishing it.
Nayef Aguerd jumped highest for a cross and goalkeeper Lucas Chevalier came out and missed, allowing the Moroccan centre back to nod the ball into the empty net.
PSG had most of the possession but the home side were more dangerous and were unlucky to go in only one up at half-time.
Amine Gouiri's shot crashed off the crossbar after 25 minutes and seconds later another goal was ruled out for offside.
The second half was a repeat of the first with PSG having most of the ball but unable to break down a stuffy Marseille defence, for which keeper Geronimo Rulli excelled.
"We should have won," PSG midfielder Illia Zabarnyi told broadcaster DAZN. "We controlled the game, played good, just maybe something was missing."
Marseille celebrated the three points with gusto, even after coach Roberto De Zerbi was sent off after exchanging words with the referee.
The win extended Marseille's fine home league start to three wins in three games and moved them up to sixth.
PSG coach Luis Enrique had the ironic consolation, at the final whistle, of learning he had been named men's manager of the year at the Ballon d'Or ceremony in Paris.
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