Matuidi, 30, has signed a three-year deal in Turin with the Italian club saying they had paid 20 million euros ($23.5 million, £18.2 million) plus a potential 10.5 million euros more in bonuses.
Matuidi, capped 58 times by France, had joined PSG from Saint-Etienne in July 2011 right at the start of the Qatari takeover which revolutionised the club.
He played almost 300 times for PSG but has slipped down the pecking order this year.
He almost left the club last year and now moves on with PSG needing to recoup some of their world-record 222 million-euro investment in Neymar.
“We wish him full success for his first challenge abroad and all the success possible with the France team in this World Cup year,” said PSG president Nasser Al-Khelaifi in a club statement.