Cavani, the top scorer in Serie A last season, is the latest big catch for Qatari-backed PSG who have spent more than 200 million euros in transfers over the past two years.
The 26-year-old Cavani scored 78 league goals in three seasons with Napoli.
PSG did not disclose the fee which media reported to be a French record 64 million euros ($83.50 million).
($1 = 0.7664 euros)
(Reporting by Gregory Blachier; editing by Toby Davis)