After Marseille lost at Reims 3-1 earlier Saturday, PSG played their part then could have clinched the title if Monaco drew at home to Riviera rival Nice in the late game.
They were 1-1 until the 73rd minute when Monaco scored again and won 2-1.
Monaco moved up to second place and were 21 points behind PSG with seven games left, meaning PSG needs only a draw at home to Angers next Saturday to again be crowned as the French champions.
PSG fell behind early to a well-worked goal when Swedish forward Lucas Stassin headed in Zuriko Davitashvili's left-wing cross.
But after Gonçalo Ramos equalised with a late first-half penalty, 17th-placed Saint-Etienne collapsed after the break. Khvicha Kvaratskhelia fired PSG ahead and Désiré Doué scored twice either side of a João Neves goal.
The 17-year-old substitute Ibrahim Mbaye completed the rout in the 90th minute with his first goal for PSG.
In the late game, Monaco striker Mika Biereth had an early penalty saved by goalkeeper Marcin Bułka and Nice took the lead shortly before the break when Jérémie Boga headed in a cross.
Biereth equalised in the 55th after latching onto Maghnes Akliouche's clever flick and shooting into the top right corner for his 12th league goal.
The lively Akliouche then showed good skill to set up Breel Embolo in the 73rd. US striker Folarin Balogun went on as a late substitute for Monaco after recovering from shoulder surgery and almost scored.
Earlier, Marseille dominated before falling behind in the 29th. Japan forward Keito Nakamura wrong-footed defender Leonardo Balerdi inside the penalty area and curled the ball in for his ninth league goal.
Careless play led to the second goal as Reims broke from midfield and Nakamura shrugged off Valentin Rongier before setting up Mamadou Diakhon for a left-foot finish.
Valentin Atangana added the third and Rongier pulled one back for inconsistent Marseille, who slipped to their fourth defeat in five games.
Marseille midfielder Adrien Rabiot was scathing about the attitude of some of his teammates.
"It's just not good enough. I get the impression of watching guys who don't want to play in the Champions League next season," Rabiot told broadcaster beIN SPORTS. "If some players don't want to, then they should say so before the game and just not play."
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