Fatik Atik netted an 86th-minute equaliser for Guingamp at the Stade Louis II, which came not long after Dimitar Berbatov had fired Claudio Ranieri's men ahead.
Guingamp lost Moustapha Diallo to a red card late on but Monaco could not find the winner they needed to keep the title race going for at least a while longer.
Ranieri's men came into the match eight points behind PSG with only three games to go and knowing anything other than victory would see them concede the league.
The newly-crowned Ligue 1 champions could not cap their title triumph with a victory, though, as they lost 2-1 at home to Rennes.
Rennes spoiled the party and all but mathematically secured their own survival as Foued Kadir and Paul-Georges Ntep cancelled out Ezequiel Lavezzi's early goal.
The champions took less than three minutes to open their account when Edinson Cavani dispossessed Sylvain Armand to release Lavezzi and the former Napoli man did the rest with a neat finish from inside the area.
Nelson Oliveira offered an immediate response but his curling effort was patted down by Salvatore Sirigu before Lavezzi went close again at the other end, dragging a 12-yard effort the wrong side of the right post.
Despite dominating the opening 20 minutes, Laurent Blanc's side were offered a timely reminder of Rennes' capabilities when Oliveira burst through on goal but the Benfica loanee saw his effort saved by Sirigu's foot with the goal at his mercy.
Soon after, though, Phillipe Montanier's men drew level when Kadir found space in the right of the area before firing a powerful shot beyond Sirigu to temporarily silence the celebratory Parc des Princes.
And those celebrations were all but quelled just four minutes later when Ntep's curling free-kick evaded everyone before bending just inside the right post with the surprised Sirigu well beaten.
PSG put their visitors on the back foot straight after the interval and Cavani should have levelled on 49 minutes when he got on the end of Javier Pastore's deft pass but the Uruguayan put the ball wide of the left post with only Benoit Costil to beat.
Yohan Cabaye was next to go close with a well-driven shot from distance which Costil gathered comfortably before the loudest cheer of the night came when Zlatan Ibrahimovic was introduced for his first appearance in seven games following injury.
The talismanic Sweden international was, as expected, involved soon after when his stabbed effort was saved by Costil.
Pastore then saw his follow-up kept out on the line by former PSG favourite Armand, before Blaise Matuidi had a solid penalty shout turned down all in the space of just a few seconds.
Ola Toivonen was gifted an opportunity to wrap up the points with just under 10 minutes to play but the unmarked former PSV Eindhoven striker blasted over the bar after being picked out inside the area by Ntep.
There was still time for Alex to find space inside the area but his header from Maxwell's delivery cannoned back off the crossbar ensuring PSG suffered their first league defeat at the Parc des Princes since losing to Rennes by the same scoreline in November 2012.