PSG move to verge of Ligue 1 crown
After second-placed Marseille crashed 3-1 to Reims earlier, PSG will win the league with seven games still to go if Nice draw at third-placed Monaco in the day's final match.
The visiting Parisians were stung after nine minutes when Lucas Stassin headed in Zuriko Davitashvili's cross to put Saint Etienne ahead.
Goncalo Ramos brought the reigning champions level from the penalty spot just before half-time, slotting home after he had been tripped in area by Florian Tardieu.
PSG took control early in the second half when Khvicha Kvaratskhelia intercepted a misplaced back pass on the halfway line and tore towards the Saint Etienne goal before shifting the ball to his left foot to score.
As Saint Etienne's effort level dropped, Desire Doue notched a double for PSG, either side of a goal from Joao Neves.
Substitute Ibrahim Mbaye completed the job with PSG's sixth goal, side-footing home from Bradley Barcola's cross.
Marseille slumped at Reims where Japanese international Keito Nakamura put the home side ahead midway through the first half before Mamadou Diakhon and Valentin Atangana added to the visitors' misery.
Valentin Rongier scored a spectacular goal to give Marseille hope in the final 15 minutes, but Roberto De Zerbi's side lost further ground on leaders PSG and are now 22 points behind in second.
Reims went ahead when Nakamura turned in the box and fired into the bottom corner for his ninth league goal this term.
Marseille dominated as the match progressed but as they pushed forward, Reims broke on 51 minutes to double their lead.
Nakamura turned provider this time, galloping onto Junya Ito's pass and prodding the ball to Diakhon, who rounded the last defender before finishing emphatically.
The relegation battlers then had De Zerbi scratching his chin in disbelief when they netted their third.
Reims struck on the counter again as Atangana raced onto Hafiz Ibrahim's through ball and slotted past Geronimo Rulli.
On 78 minutes, Rongier received the ball near the Reims box and surged forward before smashing the ball home.
Marseille midfielder Adrien Rabiot could not hide his displeasure at his teammates after a defeat which leaves them in an increasingly tight scrap for the Champions League places.
"It hurts to show this kind of spirit. I feel like I'm seeing guys who don't want to go into the Champions League and don't want to fight to get a taste of that," Rabiot told BeIN Sports.