Goals from Nuno da Costa and Stephane Bahoken gave the home side all three points against the Ligue 1 giants, who had levelled through Kylian Mbappe in the first half.
It is PSG’s first defeat since a 3-1 loss to Nice on April 30, after a run of 25 matches in all competitions, and means their lead at the top will be cut to seven points if Marseille beat Montpellier on Sunday.
Head coach Unai Emery started without Edinson Cavani, Marco Verratti or Thiago Silva in the line-up, with a Champions League clash against Bayern Munich looming on Tuesday, and they found it difficult to get going against Thierry Laurey’s well-drilled side.
It was Nuno who broke the deadlock, steering a fine header home from an unmarked position in the penalty area, but PSG improved before half-time and levelled thanks to Mbappe’s fifth league goal of the season.
Javier Pastore hit the post as PSG looked for a second but it was Strasbourg who snatched the decisive goal through Bahoken, and the home side, who went through bankruptcy only six years ago and were playing in the third tier as recently as 2016, held on for a famous victory that lifts them to 14th.