Skriniar has signed a five-year contract lasting until the summer of 2028.
The Slovakia national team captain spent six years at the San Siro after joining from Sampdoria in the summer of 2017 and made 246 appearances, scoring eleven goals.
He won one Serie A title, the Coppa Italia on two occasions and the Italian Super Cup twice.
"I am very happy to be part of this wonderful club. PSG are one of the strongest clubs in the world with world-class players and fantastic fans," he told PSG's website after his signing was announced.
Skriniar will augment a PSG defence that has lost the services of the experienced Sergio Ramos.
The French champions announced on Wednesday the sacking of Christophe Galtier as manager and have since replaced him with Luis Enrique.
