The Italy international joins La Dea for a fee of €20m up front plus €5m in bonuses.
Bellanova has signed a deal with Atalanta to 2029.
The club announced: “Atalanta BC are glad to announce the permanent signing of footballer Raoul Bellanova from Torino.
"The 24-year-old wingback native of Milan joins the Nerazzurri after imposing himself as one of the best assist providers of the 2023/24 Serie A season.
“The Percassi and Pagliuca families and the whole Club would like to extend a warm welcome back to Raoul and wish him the best of luck in his future personal and team endeavours as a Nerazzurro.”