PSG, champions for the past four years to take their overall haul to six -- the Qatari-owned outfit was only founded in 1970 -- huffed and puffed in a dire opening period before being caught by a two-goal salvo after half-time.
Angel di Maria and Uruguayan striker Edinson Cavani did their best to rouse a sluggish PSG in the opening 45 minutes but could not find a breakthrough on the way to what was the club's fourth league loss of the season.
"I think the team played to win it and controlled the game in the first half. But their second goal did it for us," PSG coach Unai Emery said afterwards.
Midfielder Marco Verratti was blunt as he lamented the reverse.
"We have lost a lot of matches -- something's not working very well. The only thing to do is not to talk but work harder. We are surely stronger than Guingamp -- if we play at 100 percent then we are going to win.
"If we're third it is not because we are playing well."
The win delighted Guingamp counterpart Kombouare, who PSG sacked five years ago to make way for Carlo Ancelotti even though they were top of the table at the time.
The big-spenders lost out to Montpellier that season but have dominated the league ever since.
"Perfect -- we won," smiled Kombouare, whose side moved fourth in the table.
"We sent out an attacking side and I congratulate my players -- we had to be very disciplined. We scored two very fine goals. We've not stolen points."
Like PSG, Guingamp reached the League Cup quarter-finals in midweek and that success over Lyon underlined the western club's league form which had taken them into the top six before start of play.
After the break, it was Guingamp who seized the initiative and they went ahead on 66 minutes as Yannis Salibur fired home inside the box.
By then Di Maria had exited the action as the Argentine became ever less effective, Lucas Moura coming on for the former Manchester United man.
Four minutes later it was 2-0 as midfielder Nill de Pauw finished off a flowing move from close range with the PSG back four all at sea at the Stade Roudourou.
Marcus Coco found Jimmy Briand and he fed De Pauw for a neat clipped finish.
Belgian De Pauw was starting for the first time having scored three times from the bench for Antoine Kombouare's men earlier in the campaign.
Two goals behind, PSG tried belatedly to rally but Cavani had a goal ruled off for offside with 12 minutes remaining.
Two minutes later he did reduce the deficit with a close-range finish for his 17th league goal of the season -- but that was as good as it good for the Parisians.
Nice top the table with 40 points from 17 games and will expect another win bonus Sunday at home to modest Dijon.
Monaco, who Sunday host a Lyon side dropping overnight to fifth, are second a point off the pace with PSG three points behind them.
Saturday's match between Caen and Metz was postponed to Sunday owing to fog.
In other Saturday action, Rennes missed a golden chance to move fourth when they slumped to a 2-1 home loss to Bastia, who exited the bottom five.
Toulouse, also targeting the top five, likewise bemoaned dropped home points -- in their case in a 1-1 stalemate against Nancy.
And Saint Etienne saw hopes of keeping in touch with the leading pack take a knock with a 2-1 defeat at Lorient as the Bretons pushed Caen down into bottom spot to close within a point of fourth-bottom Dijon.