The defender moves to Roma for a €2m loan with obligation to buy for €13m and a further set of bonuses up to €8m.
There is also a 10 per cent sell-on fee that will go to Atalanta.
“It's not a case of leaving Atalanta, it's more that Roma are a big club and it is difficult to turn down a call from them. I had two important years in Bergamo, we managed to qualify for the Champions League – but Roma is a big opportunity that doesn't come around often and I wanted to take it now and get started on this new adventure.
“I was impressed by (Paulo Fonseca), it's not every day that you get a call from a Coach as successful as him. So I can only say that I can't wait to get started."