The Brazilian, 23, was installed as a top transfer priority at Stamford Bridge in January after interest in Juventus' Alex Sandro cooled.
And Emerson has finally been announced as the Blues' second signing of the window after Ross Barkley, in a deal Roma claim is worth £17.5 million as well as a possible £7.8m in add-ons.
"Emerson Palmieri has today become the latest addition to the Chelsea squad after completing a transfer from Italian side Roma," the club announced on its official website.
"The 23-year-old has signed a four-and-a-half-year contract and will wear the No.33 shirt.
"Emerson, full name Emerson Palmieri dos Santos, arrives following a fruitful and productive two-and-a-half seasons with Roma, where he established himself as an important member of the side and gained invaluable experience of playing in Europe.
"A versatile left-sided player who can operate as a full-back, wing-back or in a more advanced role, Emerson is skillful, possesses a good range of passing and reads the game well for a player so young.
Also speaking to the club's official website, Emerson expressed his delight in getting the chance to play in the Premier League.
"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," the full-back said.
"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."
Emerson is one of two Roma players to be pursued by Chelsea this winter.
Former Manchester City striker Edin Dzeko was also a target, but Chelsea appear to have given up the chase as attentions have shifted to Arsenal's Olivier Giroud.