He jetted into west London for a medical on Thursday and, with all of the formalities completed, Torino confirmed that he has now linked up with the reigning Premier League champions.
Zappacosta is a player that Antonio Conte knows well, having handed the 25-year-old his first place in an Italy squad in 2016 during the manager's spell in charge of the national side.
He now becomes the 14th new face at Stamford Bridge this transfer window, with the highest-profile signings including Alvaro Morata, Tiemoue Bakayoko and Antonio Rudiger.
"It is a dream for me to join Chelsea and play in the Premier League," Zappacosta told Chelsea's website. "I am very excited to be here and start working with the team. I hope I can show my quality to the coach, my team-mates and the supporters."
He has to date made four appearances for the Azzurri, making his debut shortly after Conte left for Chelsea in a World Cup qualifier against Liechtenstein.