He will be joined at Stamford Bridge by Kepa Arrizabalaga. The goalkeeper is on the verge of a world record move to Chelsea after his £71million buy-out clause was met.
Athletic Bilbao confirmed this afternoon the 23-year-old's release clause has been activated and his contract terminated, although there is no mention of Chelsea.
The fee is a world record for a goalkeeper, surpassing the £65m paid by Liverpool to Roma for Brazil goalkeeper Alisson Becker last month.
The Blues have made Kepa their number one target to replace wantaway Belgium goalkeeper Thibaut Courtois.
Chelsea transfer news: Mateo Kovacic is having a medical today (Image: GETTY)
Chelsea transfer news: Mateo Kovacic starred at the World Cup for Croatia (Image: GETTY)
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A statement on Athletic's official website read: "On Wednesday, August 8, 2018 at 1202 hours (CET), the Professional Football League confirmed to us that the player, Kepa Arrizabalaga, has met the requirements for the valid unilateral termination of the work contract that linked him to Athletic Club.
"The amount of the compensation established in the contract has been deposited.
"Athletic thanks the player for his contribution during the time he has remained in our club."
Courtois has been absent from training this week ahead of Thursday's closure of the transfer window for Premier League clubs.
Chelsea transfer news: Mateo Kovacic is set for a Blues medical (Image: GETTY)
The 26-year-old, named the best goalkeeper at the World Cup as Belgium finished third, has one year remaining on his contract and reportedly wishes to join Real Madrid.
Courtois spent three years on loan at Atletico Madrid from Chelsea, before becoming the Blues' number one in 2014-15.
Real head coach Julen Lopetegui refused to comment on Courtois after his side's friendly win over Roma in New Jersey, insisting that Keylor Navas remains his number one.
Chelsea boss Maurizio Sarri declined to discuss Courtois or Kepa after his side's final pre-season friendly on Tuesday night, against Lyon at Stamford Bridge.