Lens lose, giving PSG chance to reclaim Ligue 1 lead
Odsonne Edouard fired in the opener from an Adrien Thomasson lay-off after just three minutes at the Stade Bollaert-Delelis, where Lens were looking for a club record 11th successive home league win.
They were on course to get the victory, and move provisionally four points clear at the top, when Florian Thauvin doubled their advantage on 56 minutes.
However, Monaco pulled one back just after the hour mark courtesy of Folarin Balogun, with the United States star scoring his 11th goal of the season in all competitions.
That sparked a stunning comeback, with Denis Zakaria equalising before substitute Ansu Fati netted the winner with a third goal in the space of 11 second-half minutes for the principality side.
It was a wholly unexpected capitulation from Lens, who came into the weekend with the best defensive record in the division.
PSG lie a point behind Lens in second and have a better goal difference, meaning Luis Enrique's team will be back on top of the table provided they avoid defeat at home to bottom club Metz later.
The match at the Parc des Princes comes in between the two legs of PSG's Champions League knockout phase play-off against Monaco, who head to the capital on Wednesday for the return after losing 3-2 at home last Tuesday.
Monaco are provisionally up to sixth, right back in the mix for European qualification having won three and drawn two of their last five Ligue 1 outings.
Toulouse play Paris FC in Saturday's other game in France, after Marseille lost 2-0 at Brest on Friday in new coach Habib Beye's first match in charge.