The 25-year-old Spanish left-back is the first defender signed by Antonio Conte following the recruitment of a forward, a midfielder and a goalkeeper to his squad. He has signed a five-year contract.
Speaking, 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.”