The move is originally a loan agreement but includes an obligation to make the deal permanent in the summer for €6.75 million with €5m in potential add-ons according to ESPN sources.
Reynolds, 19, is the latest American player to sign for a European club after Roma beat competition from clubs across the continent to sign him.
"I love this game so much and I'm excited for my future," Reynolds said. "I grew up with FC Dallas and wouldn't be where I am if I didn't have the opportunity with the club. I'm going to fight and give everything to Roma like I did here. I would like to thank the fans and FC Dallas for their support. I'm just super excited for my future and I'm going to learn Italian as fast as possible."
Sources told ESPN that Roma's Serie A rivals Juventus were confident of landing Reynolds earlier in the transfer window and planned to loan him to another Serie A side for six months as they had already signed their two permitted non-EU players this term.
🤝 Welcome to #ASRoma, Bryan Reynolds! 🇺🇸— AS Roma English (@ASRomaEN) February 1, 2021
As a club we are proud to continue supporting missing children charities around the world - working together to improve the chances that youngsters across the globe will be returned home safely. ♥️ pic.twitter.com/JaM9oK0Ea3
Reynolds would have been the second American on Juve's books -- joining fellow FCD academy product Weston McKennie, himself on loan for the season from Bundesliga club Schalke 04. The Bianconeri have the option to make the McKennie's deal permanent at the end of the season.
But Roma came back in with an improved offer for the player, who also attracted attention from Belgian side Club Brugge, and he will join Paulo Fonseca's team sitting third in Serie A.
Reynolds is yet to make his USMNT debut but has regularly featured for the U.S. at youth levels. He was named to the U.S. Under-23s training squad earlier in January but withdrew for personal reasons.