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Forward Scamacca, 24, has signed a four-year contract with the Bergamo club on a deal worth a reported 25 million euros ($27.4 million).
"West Ham United would like to thank Gianluca for his efforts and wish him well for his future career," the English club said.
Scamacca joined West Ham last summer for 36 million euros from Sassuolo but his debut season was overshadowed by injury and difficulty adapting.
He scored eight goals in 27 matches for the Premier League side who finished 14th in the table, including five in the Hammers' run to the UEFA Conference League success.
In Bergamo, he replaces Denmark's Rasmus Hojlund who signed for Manchester United for 74 million euros.
Atalanta finished fifth in Serie A last season.