Marcos Alonso has joined Chelsea from Fiorentina on a five-year contract in a deal reportedly worth around £23 million.
Fiorentina confirmed the deal on their official website on Wednesday afternoon in a statement, which read: "ACF Fiorentina announce that they have sold, on a permanent deal, the sporting rights to the footballer Marcos Alonso Mendoza to Chelsea Football Club."
Chelsea later confirmed the deal and Alonso said: "I'm very happy. It's a step up in my career and I think this is the perfect place for me to continue developing. It's time for me to win some titles.
"It was an easy decision for me to make when I heard the club were interested in signing me. My family and close friends know Chelsea is the club I have dreamed about playing for so I'm really excited about the challenge and I'm looking forward to working with my new team-mates."
Antonio Conte has prioritised the signing of a left-back and a centre-back in the final weeks of the transfer window and Alonso has experience in both positions, though he was deployed most regularly on the left of midfield by Fiorentina last season.
Alonso becomes Chelsea's fourth senior signing of the summer, joining Michy Batshuayi, N'Golo Kante and Portugal goalkeeper Eduardo at Stamford Bridge, and Conte is still chasing a centre-back before the transfer window closes on Wednesday.
Sources told ESPN FC on Tuesday that Paris Saint-Germain had rejected an offer of around £35m from Chelsea for David Luiz, but talks between the clubs are ongoing over the possibility of bringing the Brazilian back to Stamford Bridge.
Chelsea have also reportedly enquired about the availability of West Ham United defender Angelo Ogbonna, who worked with Conte at Juventus.
The Blues have been frustrated in their pursuit of a number of Serie A defenders this summer, and sources told ESPN FC on Tuesday that top target Kalidou Koulibaly has given up hope of moving from Napoli to Stamford Bridge in this transfer window.