Unai Emery's side, who have taken only one point in their last three league games, are third with 36 points from 18 games after goals by Yannis Salibur and Nill De Pauw gave them their fourth defeat of the season.
Edinson Cavani pulled a goal back for PSG 10 minutes from time.
Guingamp are now fourth on 29 points.
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Leaders Nice, who host Dijon on Sunday, have 40 points with second-placed Monaco one point behind before taking on Olympique Lyonnais, also on Sunday.
Saturday's game at the Roudourou was sweet revenge for Guingamp coach Antoine Kombouare, who was sacked by PSG in Dec. 2011 to make way for Carlo Ancelotti while the club were Ligue 1 leaders.
"The only thing to do is to stop talking and work in silence," said PSG midfielder Marco Verratti. "If we're only third it means that we are not playing well."
PSG enjoyed possession but were toothless up front in a dull first half.
Their best move came in the 25th minute when Cavani's volley from Angel Di Maria's long cross went just wide.
Holding midfielder Thiago Motta was far from his best but Guingamp failed to capitalise on the Italian's poor performance.
After a fine one-two with Marco Verratti, Cavani was denied by Karl Johan Johnsson from close range as PSG stepped up a gear after the interval.
Guingamp, however, were looking to hit PSG on the break and they did just that in the 66th minute when Salibur beat Alphonse Areola with a low shot after being set up by a superb through ball from Lucas Deaux.
Another counter attack and another goal came for the hosts four minutes later, De Pauw wrapping it up from close range.
Cavani reduced the arrears from close range from Layvin Kurzawa's cross for his 17th goal in Ligue 1 this season.
PSG pushed hard in stoppage time but Johnsson denied Cavani from point-blank range to keep his side ahead.
(Reporting by Julien Pretot; Editing by Clare Fallon)
