Laurent Blanc's men went into the game at Stade du Roudourou knowing a win would put them eight points clear of second-placed Monaco ahead of their clash with Olympique de Marseille on Sunday.
Instead the visitors were frustrated by an industrious and organised Guingamp side, who claimed a fourth draw in their last six top-flight games.
Legitimate goalscoring opportunities proved few and far between throughout the contest, although Edinson Cavani did have a goal disallowed early in the second half for offside.
Yet it was Guingamp who went in front inside the final 10 minutes as Mustapha Yatabare scored his sixth league goal of the campaign.
However, Jocelyn Gourvennec's men were denied a famous victory, Alex levelling matters to extend PSG's lead at the summit to six points.
Brazilian Marquinhos replaced injured compatriot Thiago Silva in the centre of the PSG defence, while Guingamp made four changes to the side that lost 2-0 to local rivals Lorient last weekend.
Both sides went close with efforts from distance in the early going, Yatabare shooting wide for the hosts before Zlatan Ibrahimovic curled narrowly over the bar for PSG.
The champions controlled play for the majority of the first half, but struggled to create chances against a resolute Guingamp defence.
Indeed, Blanc's men were reduced to shots from distance, Ibrahimovic forcing a smart save Guy N'dy Assembe with a 30-yard drive.
And PSG were fortunate not to fall behind on the stroke of half-time, as Salvatore Sirigu turned Rachid Alioui's swerving free-kick on to the underside of the crossbar with his foot.
Guingamp made a confident approach to the second half and spurned another chance to go ahead when Yatabare headed wide from Alioui's pinpoint left-wing cross.
The hosts' resistance looked to have been broken after 51 minutes, Edinson Cavani latching on to Thiago Motta's throughballl to slot home, only to be denied by the linesman's flag.
Guingamp continued to defend with relative comfort, and Blanc soon shuffled his pack in an attempt to find a winner, Lucas Moura replacing Javier Pastore after 70 minutes.
But it was Guingamp who finally broke the deadlock seven minutes from time as Yatabare scored the goal that Gourvennec's men's efforts merited.
The Mali international rose highest to power Alioui's corner into the top corner leaving Sirigu.
Guingamp's lead lasted just three minutes, though, as Alex restored parity with a fine header from x's corner to spare PSG's blushes and make it six Ligue 1 games without defeat for the champions.