Captain Marquinhos gave PSG the lead just past the half-hour mark at the Parc des Princes, where Luis Enrique's team were playing in front of their own fans for the first time in this campaign.
PSG still winless in Ligue 1 as Monaco go top
Paris Saint-Germain's slow start to the French season continued as the reigning European champions went in front only to lose 2-1 at home to Monaco, who are top of Ligue 1.
But Monaco hit back in the second half as Flavio Nazinho equalised before Belgian winger Stanis Idumbo fired in the winner for the principality side, who are top of the early standings with three wins from three.
PSG, who announced before kick-off that coach Luis Enrique had signed a three-year contract extension to stay at the club until 2030, will begin their bid to win a third consecutive Champions League title next midweek at home to Slovan Bratislava.
However, they will do so off the back of a stuttering start domestically, with just two points on the board three games into their latest Ligue 1 title defence -- that leaves them seven points behind Monaco already.
The Qatar-owned club beat Aston Villa last month to win the UEFA Super Cup, but then lost to Lens in the French Champions Trophy before drawing their first two Ligue 1 outings, away to Rennes and Lille respectively.
They had come from two goals down late on in both of those games, with Marquinhos grabbing a last-gasp leveller a week ago in Lille.
The Brazilian defender, PSG's captain, opened the scoring here on 31 minutes when he converted Khvicha Kvaratskhelia's ball across the face of goal with his chest.
PSG looked on course at that point for a welcome victory against a Monaco side coming off a turbulent week.
The principality side, now coached by Brazilian ex-Atletico Madrid defender Filipe Luis, had been hoping to make a big sale to the Premier League before the transfer window closed to offset failing to qualify for this season's Champions League.
- Balogun missing -
But they were left reeling as United States World Cup star Folarin Balogun backed out of an agreed £40 million ($54 million) transfer to Everton on deadline day.
That was just after Monaco had pulled out of a deal to sell Lamine Camara to Chelsea, when they were banking on Balogun finalising his move.
Balogun, who was at the centre of controversy at the World Cup after US President Donald Trump successfully intervened to force FIFA to suspend a red card so he could play against Belgium in the last 16, was left out here and has not played for his club this season.
Senegal midfielder Camara did feature for the visitors, who drew level on 58 minutes when Portuguese left-back Nazinho headed in at the back post from a Jordan Teze delivery.
That was Monaco's reward for a good start to the second half, and they went ahead on 72 minutes when Aleksandr Golovin supplied Idumbo and he beat Matvey Safonov at the goalkeeper's near post.
It is the second season running in which PSG have lost at home to Monaco, with the introduction off the bench late on of reigning Ballon d'Or Ousmane Dembele and Spain World Cup final hero Ferran Torres not enough to save the hosts.
Torres had a late goal disallowed for an offside against Dembele, before the latter was denied by a superb Lukas Hradecky save deep in stoppage time.
Lyon beat Auxerre 3-1 earlier and are in second place, two points behind Monaco and level on points with Lille who won 1-0 at Toulouse on Thursday thanks to an Ayase Ueda goal.
