The Brazilian's 87th-minute header cancelled out Mustapha Yatabare's effort at the other end with six minutes remaining of a hard-fought encounter played in wet and windy conditions in Brittany.
The draw is enough for the defending champions to extend their lead at the top of the table to six points, although second-placed Monaco can trim the gap back to three with a win against Marseille on Sunday.
PSG had endured a mixed week, thumping Nantes 5-0 in Ligue 1 before being surprisingly eliminated from the Coupe de France by Montpellier.
At the Stade du Roudourou, coach Laurent Blanc was without injured captain Thiago Silva, so Marquinhos lined up in central defence, while there was a first start in Ligue 1 since the beginning of December for Javier Pastore.
And against opponents who had not won in six league games and had scored just three times since the start of November, PSG for once struggled to stamp their authority on proceedings.
Zlatan Ibrahimovic had a shot from 40 yards turned around the post by Guy Roland Ndy Assembe with the visitors' best chance of the first half, but that came just two minutes before the interval.
Guingamp were a greater threat at the other end, with the 21-year-old Rachid Alioui taking advantage of a rare start to volley just wide early on and then send in a long-range free-kick that moved in the air and was turned desperately onto the crossbar by PSG 'keeper Salvatore Sirigu.
Edinson Cavani had an effort correctly ruled out for offside at the start of the second half, and the game looked to be headed for a goalless draw on a difficult pitch.
However, the home side had other ideas and took the lead in the 84th minute when striker Yatabare stole a yard on Marquinhos to head in from a right-wing corner.
It was the Malian striker's sixth league goal of the campaign, and his first since early October, and it sparked wild scenes of celebration among the packed crowd inside a stadium which boasts a capacity greater than that of the small town of Guingamp itself.
However, the giants from the capital were rescued soon after when centre-back Alex got to a corner before Ndy Assembe to head home and restore parity.
"It is always difficult to win games. We came here to win even if we had a lot of respect for Guingamp," Blanc said.
"It is disappointing but I am still satisfied that we never gave up and came back to get a deserved point.
"We were not as good today. Why? I don't want to hide behind it but the state of the pitch was deplorable. It is the case for both sides but it handicaps the team that plays the most football.
"We knew it would be difficult and we have to get on with it. It maybe won't be the last time either, but we were not exactly helped."
Monaco face season without Falcao, Lille draw
Monaco will now look to close the gap without the services of top scorer Radamel Falcao, who underwent knee surgery on Saturday after suffering a serious injury in a French Cup win at Chasselay in midweek.
Third-placed Lille could only manage a 1-1 draw at home to Rennes on Friday despite Salomon Kalou giving them the lead, and they have now gone four games without a win.
Saint-Etienne can move to within a point of the podium with a win at Bordeaux on Sunday, while in-form Lyon are also in action on Sunday against Evian.
Elsewhere on Saturday, Ajaccio and Sochaux remain cut adrift at the foot of the table after drawing 1-1 in Corsica, Vincent Nogueira scoring at the death to earn the visitors a point.
Valenciennes remain in the drop zone after coming from behind to draw 1-1 with Lorient, but Montpellier continued their climb up the table by beating Nice 3-1.
Nantes and Reims drew 0-0, while the meeting between Toulouse and Bastia fell victim to the weather.