PSG won the title by 31 points in May, losing only two games, but they were outclassed by Monaco who finished third last term.
Monaco moved to the top of the table, level with Guingamp and Nice on seven points from three matches and one ahead of PSG who won their first two games of the season.
Joao Moutinho put the home team ahead after 13 minutes and Fabinho doubled the lead with a penalty on the stroke of halftime.
Edinson Cavani pulled one back for the champions with a header but Serge Aurier's own goal 10 minutes from time secured a deserved victory for Monaco who were far more clinical than their opponents.
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(Reporting by Ed Osmond; Editing by Tony Jimenez)
