The Parisian side have endured a hard season following the resurgence of Monaco under Leonardo Jardim, and they now find themselves three points behind the league leaders, who also have a game in hand over Les Rouge-et-Bleu.
Monaco, knocked out of the Champions League by Juventus in midweek, need only a point from their remaining two games to rubber-stamp their first French crown in 17 years and halt PSG's four-year domestic dominance.
Radamel Falcao struck early on and added another in the second half after Bernardo Silva had given Monaco a 2-0 interval lead, while Junior Alonso's late own goal sealed a comfortable victory. The Brazilian winger Lucas also got two, with Julian Draxler scoring the other.
"Monaco have had a great season but they're not yet mathematically champions", said Emery, although his side's goal difference of plus-56 trails well behind Monaco's 73.
It was a sad farewell to fans for Saint-Etienne coach Christophe Galtier, taking charge of his 359th game and his last one at home before leaving at the end of the season.
The home side doubled the tally on the stroke of halftime, Silva tapping in from close range after being set up by Kylian Mbappe, who had slalomed into the visitors' defence.
And while last season's runners up Lyon are nearly assured of fourth and Europa League football next season, Marseille travel to Bordeaux in a crunch clash for the the second Europa League qualifying place - Marseille now lead Bordeaux by a point. Normally such a reserved character, Jardim jumped off the bench and pumped his fist vigorously as he shuffled along the touchline.
"We have a great collective strength", Monaco left back Benjamin Mendy said "You can only enjoy yourself when you're in this team".
Nice, guaranteed a top-three finish, came unstuck in a 2-0 home defeat to Angers, for whom Cheikh Ndoye was on target in the first half and Karl Toko Ekambi scored deep in stoppage time.
There is another crucial relegation battle as second bottom Nancy visit third bottom Dijon with just a point separating the two.