The 23-year-old Italy international joins on an initial loan deal, costing €5m, with an obligation for Roma to complete the permanent signing of the player for a further €15m if various sporting targets are met.
Cristante has agreed a contract that runs until 2023 and joins after 18 months with Atalanta, where he scored nine goals in 36 Serie A appearances for the seventh-placed side last term.
He has also featured five times for Italy since making his debut in October 2017, most recently appearing in the 1-1 draw with the Netherlands earlier this month.
Speaking to the club's official website, Cristante said: "I am very pleased because coming to Roma was always my aim.
"There was a strong willingness, both on my part and from the club, to find an agreement, and we managed to do that very quickly.
"I am confident that I will be able to do my best and make a big contribution to this club."
"With the signing of Bryan, Roma have acquired one of the most promising central midfielders in Italian football," sporting director Monchi said of the signing.
"In the future we hope he will become one of the pillars of Roma and the national team."
Cristante becomes the Giallorossi's third signing of the window, following the captures of Ante Coric and Ivan Marcano.