It was only the second loss in all competitions for PSG in the 2018-19 campaign, reports Efe news.
The hosts on Wednesday appeared to have the situation well in hand after Neymar scored on a header in the 63rd minute.
But PSG grew complacent and an aggressive Guingamp forced the home side to commit a pair of fouls that proved fatal to the Parisians' hopes of a sweep of domestic titles.
The first, in the 81st minute, was converted by Yeni N'Gbakoto, while Marcus Thuram, son of legendary French international Lilian Thuram, got the winning goal from the spot in the third minute of stoppage time.
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