The Belgium worldwide, named best goalkeeper at the World Cup this summer as he helped his country to third-place, has signed a six-year deal after completing a medical on Thursday.
Athletic said the 23-year-old Kepa had his €80-million (NZ$138m) buyout clause paid on Wednesday, although the Spanish club did not say which team made the payment. As the part of the contract, Mateo Kovacic, midfielder of the Spanish powerhouse, will join the blues on a season-long loan.
Liverpool are the window's biggest spenders so far, with boss Jurgen Klopp bringing in £160 million of new recruits, but the total spend by Premier League clubs is unlikely to smash the £1.4 billion record set in 2017.
Courtois, who played three years with Atletico, said he still isn't in top form following the World Cup, but will be ready to play in the Super Cup if needed.
He later deleted the post but again thanked everyone involved with Chelsea once he had completed the move to Madrid.
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Belgium worldwide Courtois, 26, has yet to return to training and is linked with a move to Real Madrid.
Kepa was close to joining Real for £17.9m in January, only for Zinedine Zidane to cancel the deal because he felt that signing a goalkeeper during the season would be disruptive. Kovacic, who had a two-year stint in Italy with Inter Milan, has also said he is looking forward to sharing the pitch with his fellow new signing.
While not discussing the status of the transfer, Chelsea coach Maurizio Sarri said that Kepa was "very young, but very, very good".
He's not wrong. Kovacic is likely to take game time away from encouraging youngsters such as Ruben Loftus-Cheek.
But it might not have happened had it not been for former Liverpool and Napoli goalkeeper Pepe Reina, who Kepa has cited as the reason that he made a decision to join Chelsea.
"When I [first] saw him play, I was just so struck by the fact that a keeper who was then only 18 could perform like that under such pressure".