The 27-year old was a stalwart in Leicester's midfield over the past two seasons, proving reliable in a title-winning campaign and bringing a sense of calmness during last season's step backwards.
He also said it was "a dream for me to join Chelsea" after his deal was finalised. "I hope I can show my quality to the coach, my teammates and the supporters", Zappacosta told the club's official website.
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.
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She was defeated in the 135-pound championship bout by Holy Holm, who knocked Rousey out with a kick to the head. UFC was interested in her making a return to the Octagon this year but there has been no talk of it of late.
The Italian connection is clear in the Zappacosta deal as it was Antonio Conte who gave the full-back his debut for Italy in 2016.
The pair met during his time playing for Avellino, the 23-year-old's native city where she still works in an electronics shop, and are used to having a distance relationship after his return to Atalanta then Torino.