The transfer window concluded with a frantic deadline day that saw many of the top European sides make a number of late buys to bolster their side for the 2016/17 season.
The 25-year-old Spaniard is the first defender to be signed by Antonio Conte for Chelsea.
Fiorentina sporting director Carlos Freitas has denied speculation of a Chelsea bid for wing-back Marcos Alonso a day after reports surfaced that the Blues have entered into advanced negotiations with the Viola for the Spaniard.
Chelsea are edging closer to acquiring a defender after missing out on a number of key targets this summer, with the signing of Fiorentina fullback Marcos Alonso reported to be imminent.
Chelsea are looking for a left-back as well as a centre-back in Italy and latest reports suggest that the Blues are talking to Fiorentina in order to sign former Bolton Wanderers left-back Marcos Alonso, who has had two excellent seasons with the Viola.