Cedric Bakambu netted the winner six minutes from time as PSG conceded three goals in a league game for the first time this season.
Carlo Ancelotti's side looked nothing like the team who beat Valencia 2-1 at the Mestalla in a Champions League last 16 first leg clash on Tuesday as Sochaux eased their relegation fears with an impressive performance.
PSG had won 12 of their last 13 matches in all competitions - eight of their last nine league games - but they lacked inspiration throughout and missed the injured Jeremy Menez.
They stay top with 51 points from 25 games, but their lead was reduced after second-placed Olympique Lyon hammered Girondins Bordeaux 4-0 away earlier on Sunday.
"This week's game took its toll physically," midfielder Blaise Matuidi, who put in a below-par performance, told French TV channel Canal Plus.
"Everyone was at fault. We now have to question ourselves and get back on our feet."
PSG were without new signing David Beckham at Sochaux's Stade Bonal, as the former England captain has yet to regain full fitness since joining the Ligue 1 club at the end of the January transfer window.
The midfielder is likely make his debut at the Parc des Princes against Olympique Marseille next Sunday.
Sochaux started brightly and Giovanni Sio had a goal disallowed for offside in the fifth minute.
PSG, however, held on despite losing the midfield battle, and opened the scoring in the 29th minute when Alex headed home from a Javier Pastore corner.
Sochaux, who moved up to 14th on 26 points, hit back seven minutes later, with Sebastien Roudet firing an angled shot past Salvatore Sirigu after a brilliant charge down the left flank.
Sochaux maintained the upper hand after the interval and deservedly took the lead in the 54th. Sio played a fine one-two with Roudet and beat Sirigu with a low shot.
The home side came close to a third on the hour as Sebastien Corchia's 25-metre free-kick smashed the post, but PSG centre-back Mamadou Sakho equalised with a deflected shot in a packed goal mouth.
The hosts, however, claimed all three points in the 84th minute. Ryad Boudebouz's cross to the far post was deflected by Sloan Privat into the path of Bakambu who poked the ball into the empty goal.
Clement Grenier scored a double as Lyon, who had lost their last three games in all competitions and conceded eight goals in the process, revived their title hopes by hammering Girondins Bordeaux 4-0 away.
Grenier collected a Bafetimbi Gomis through ball and kept his composure to fire it past Cedric Carrasso after 15 minutes.
Bordeaux barely threatened and fell two goals behind when Gueida Fofana unleashed a shot straight into the left top corner 20 minutes into the second half.
The hosts had only themselves to blame for the third goal as Julien Faubert, back at Bordeaux after six years away, fouled Fofana in the box.
Grenier converted the resulting penalty in the 73rd minute and a volley from Alexandre Lacazette two minutes later sealed the one-sided
