Juventus said the deal included an option to buy for €7 million, payable in three years, with further performance-related add-ons worth two million euros.
The club paid a loan fee of two million euros to sign the striker from the French club.
Juventus were looking to strengthen their frontline with the addition of Barcelona forward Memphis Depay, Italian media had said, but they have ended their pursuit of the Netherlands international due to Barca's financial demands.
Milik returns to the Italian Serie A where during his 4-1/2-year spell at Napoli he made more than 120 appearances in all competitions and scored nearly 60 goals.
He joined Marseille in January 2021 on an 18-month loan with an obligation to buy, a clause which was executed in the close season. By that time, he found the back of the net 30 times for the French side.
Milik has earned so far 62 international caps and scored for Poland 16 goals.
At Juve, he joins fellow Poland international, goalkeeper Wojciech Szczesny, who made the permanent switch to Serie A from English Premier League side Arsenal in 2017, following a loan spell at Roma.
