Emerson has signed a four-and-a-half year contract that will see him remain a Chelsea player until the summer of 2022.
23-year-old Emerson has been with Roma since 2015 after leaving his native Brazil. He is a product of the famed Santos youth system and got his first taste of football in Europe when he joined Palermo on loan in the latter half of the 2014/15 season.
Now an Italian citizen through heritage on his mother's side of the family, Emerson has missed much of the season so far as a result of a serious knee injury.
It ruled him out of both clashes with Chelsea in the Champions League group stage and he has managed just 15 minutes of Serie A football since returning to the squad at the start of November while he regained fitness.
He said on the move: "I came here because it’s a great team with a great history. Today I am realising one of my dreams to pull on the shirt of such a great club.
"English football is something I have been watching since I was 15 years old. It is a really beautiful game over here but it is going to be even more beautiful to play it."
Chelsea director Marina Granovskaia added: "We are delighted to welcome Emerson to the club. He is a young and talented player who strengthens the squad and will provide excellent competition for places as we pursue success in three different competitions."