Monaco needed all three points from their clash on Wednesday to keep the title race alive, ahead of PSG's encounter with Rennes later in the day, but they were frustratingly held by a determined Guingamp side, who earned a draw late on.
The principality club looked to be on their way to victory when Dimitar Berbatov headed home Nabil Dirar's right-wing cross after 77 minutes.
But Claudio Ranieri's men were stunned just eight minutes later as the Coupe de France winners levelled, Fatih Atik finding the net from just inside the area with a fine volley past Danijel Subasic.
Monaco were given renewed hope when Mustapha Diallo was sent off for Guingamp in the closing stages following a collision with Jeremy Toulalan.
However, the hosts could find a way past the 10 men, who clung on to hand PSG their second successive Ligue 1 title.
