Zalewski, 23, was borrowed by Roma in January to Inter for the second half of last season. The transfer became permanent in June.
Inter spent € 7 million to sign Zalewski downright.
La Dea has now signed the wingback for € 17 million, with the deal confirmed this morning.
Zalewski is now being reunited with the new Atalanta coach Ivan Juric, with the couple for a short period of last season at Roma.