Emerson has signed a four-and-a-half year contract with the west London side and he will wear the number 33 third for Antonio Conte's men.
The Blues had been linked with the Brazilian for most of the window and initially it was believed that he would join along with Roma team-mate Edin Dzeko.
Chelsea remain open to bringing in Dzeko but are sounding out alternatives as the Serie A outfti have become harder to negotiate with amid fan pressure to keep their star striker.
The Roma defender has only recently recovered from an anterior cruciate ligament injury, but is expected to pass his medical after recently returning to the pitch to make two appearances this season.
"Now I am definitely going to improve here at Chelsea under Antonio Conte", Emerson added.
"Yes, physically I feel great", he told the club's official website.
"He is someone who has already shown that he is a great coach and everyone who plays with him improves, so I am coming to Chelsea with this perspective, that I am going to be playing with one of the best coaches in the world and I am going to get better".
Watford signed Belgium Under-21 winger Dodi Lukebakio on a 4 1/2-year deal and allowed Mauro Zarate to join Velez Sarsfield on loan until the end of the season.
'English football is something I have been watching since I was 15 years old.
Pressed again on whether he had seen much of Emerson, Conte continued: 'I don't remember Palmieri because he played in Italy only last season.