A Chelsea statement added: "Quick and agile, Kepa possesses fantastic reflexes and has developed his all-round game since establishing himself as Athletic Bilbao's first-choice 'keeper two years ago".
The Spain worldwide, 23, has signed a seven-year contract at Stamford Bridge and joins the Blues for £71m - a world record fee for a keeper.
Eden Hazard has hinted he is keen to leave Chelsea this summer as he eyes up a move to Real Madrid, but Blues officials have told Yahoo Sport that they will not entertain any offers for their number 10.
Courtois has been absent from training this week before Thursday's closure of the transfer window for Premier League clubs.
Hiroshima marks 73rd anniversary of atomic bombing in WWII
Tens of thousands of people gathered to honour the victims at an annual memorial ceremony held in the city. But in order to bring this treaty into effect, 50 nations must ratify it, of which only 14 have done so.
"His long-term contract reflects the belief we have in him".
Chelsea open their Premier League campaign at Huddersfield on Saturday, having suffered a 2-0 defeat to Manchester City in the Community Shield last weekend.
"It's a very important decision for me, for my career, and also for my personal life", said the goalkeeper.
In a 90-minute spell, Chelsea agreed to sell Belgian goalkeeper Thibaut Courtois to Real Madrid in a deal worth around 35 million euros.
The seven-year deal certainly puts Chelsea in a position of strength which had been lacking with outgoing Courtois.
The Basque club on Wednesday confirmed the 23-year-old's release clause of €80million (£72m) was activated and his contract terminated, although made no mention of Chelsea.