The champions succumbed to second-half goals by Yann Bodiger and Jimmy Durmaz as they dropped to third in the table with 13 points from seven games.
Toulouse are now second on 14 points, trailing Nice on goal difference before the leaders travel to Nancy on Sunday.
PSG, without Hatem Ben Arfa after the forward was omitted from the squad for the fourth match in a row by coach Unai Emery, were nervy after having Serge Aurier sent off in the 46th minute.
The visitors made a cautious start to the game but soon upped the pace and went close three times before the break.
Keeper Alban Lafont denied Edinson Cavani from point-blank range with a superb save on the half-hour.
Cavani then connected with Maxwell's 41st-minute cross following a sweeping move but Lafont parried the Uruguayan's header.
Three minutes later Aurier's header was cleared from just under the bar by defender Christopher Jullien.
Toulouse were the stronger team at the beginning of the second half and Bodiger converted a penalty in the 48th minute after Aurier fouled Ola Toivonen and picked up his second yellow card.
PSG forward Javier Pastore went on for Brazilian Lucas in the 57th minute but made little impact.
Durmaz put the result beyond doubt 11 minutes from time by beating keeper Alphonse Areola from close range after intercepting a poor Thiago Motta pass.
Monaco, handed their first defeat by Nice on Wednesday, can go top by beating Angers on Saturday.
(Editing by Tony Jimenez)
