The 22-year-old was paraded in front of Napoli fans yesterday, but the deal was not made official until this afternoon.
Milik impressed for Poland in Euro 2016 after enjoying a successful spell at Ajax. The striker scored an impressive 32 goals in 52 games for the Dutch giants.
The Naples club have been quick to invest some of the money received from Gonzalo Higuain's €90 million move to Juventus.
Napoli released the following Tweet which reads: "Official: Arkadiusz Milik is a Napoli player".
Ufficiale: Arkadiusz Milik è un calciatore del Napoli.
— Official SSC Napoli (@sscnapoli) August 2, 2016
Il comunicato: https://t.co/61DIHNPYQJ pic.twitter.com/S1X8oEdzst