The 24-year-old moves to London as part of the deal that takesThibaut Courtois to Real Madrid.

The Croatian international leaves the Spanish capital after three years at the club following his move from Inter Milan in 2015. 

Kovacic made just 17 appearances in La Liga last season and moves to the Premier League in search of regular first-team football. He also played five games during Croatia's run to the World Cup final this summer.

Kovacic told Chelsea's club website: "I am really happy and excited to be here at Chelsea. It is an amazing feeling. I will try to do my best for this club.

"It is a new league for me, the beginning is always tough but I am sure the coach and my new team-mates will help me and I am looking forward to a great season." 

Chelsea director Marina Granovskaia added: "We are very happy to welcome Mateo to the Chelsea family. He is a perfect fit for us this season, his style is well suited to how Maurizio wants to play and we are sure he will prove to be a valuable asset for the club."