The 23-year-old played just twice for Roma this season after missing the first three months of the campaign with a serious knee injury.
“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,” Emerson told the Chelsea website.
“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.”
The Brazil-born defender, who joined Roma from Santos in 2015, obtained Italian citizenship last year and was called up to the Italy squad by then coach Gian Piero Ventura in May.
But he was injured on the final weekend of the Serie A season and was forced to withdraw from the squad.
Would you like to place a soccer bet? Click here to get started in 3 easy steps.