Sofiane Diop and Guillermo Maripan were on target for Monaco, who became the first team since AS Nancy in the 2011/12 season to complete a domestic double over Paris Saint-Germain.
The hosts, who thrashed Barcelona in the UEFA Champions League in midweek, appeared to be exhausted and turned in a flat performance.
The defeat was Paris Saint-Germain's sixth of the season, their most in a single league campaign in nine years. They now trail runaway leaders Lille by four points.
Here are five hits and flops from Paris:
Flop: Kylian Mbappe (Paris Saint-Germain)
Having put up an inspired performance against Barcelona on Tuesday, netting a brilliant hat-trick at the Camp Nou, Kylian Mbappe seemed to have run out of gas against Monaco.
The 22-year-old forward's magic was nowhere to be seen as he was completely overpowered by Monaco's stoic defense, who kept him firmly under wraps.
Failing to muster even a single effort at goal, Mbappe was dispossessed four times on the night, the most alongside Moise Kean, and completed only three of his seven attempted dribbles.
The Frenchman boasts a terrific record against his former employers but tonight was arguably one of his worst-ever performances for Paris Saint-Germain.
Hit: Aurelien Tchouameni (AS Monaco)
Aurelien Tchouameni ran the show for Monaco, making his impact at both ends of the field as the visitors never ceded control of the match.
The 21-year-old was the key to breaking PSG's attack. He made several blocks and interceptions while spraying passes forward to Monaco's attackers.
The former Bordeaux midfielder has made a huge impact for Monaco in his first full season with the club, and tonight was another reminder of his seamless potential.