The 25-year-old will link up with Gian Piero Gasperini’s side initially on loan until the end of the season but the deal will be made permanent in the summer.
Boga joined Sassuolo from Chelsea in the summer of 2018 and made 98 Serie A appearances for the club in total, scoring 18 goals and providing six assists in that period.
"I'm really happy right now and so is my family as we've wanted to come here for a long time. Now we're happy that the deal is done," Boga told the club's official media.
"I think Atalanta are a great team, they are very strong and have proved to be an important team in Serie A over the past five or six years.
"I think it perfectly matches my style of play. I believe it's the right team for me to take another step and try to win something important.
"I know everybody expects something more from me and, at the same time, I expect myself to do better. I'm happy to be here.
"I know it's a solid team where everyone runs for the other, where each player works, and I think it's perfect, as you can see it's a real team."
Boga is currently away on international duty with Ivory Coast at the Africa Cup of Nations, where he is yet to make an appearance.
