Chelsea have signed Kepa Arrizabalaga from Athletic Bilbao for £71m - the highest fee ever paid for a goalkeeper, and a club record transfer.
Kepa, 23, arrives at Stamford Bridge after a release clause in his contract was met, with the deal surpassing Alisson's £66.8m move to Liverpool.
He has signed a seven-year contract at the club and replaces Thibaut Courtois, who is set to join Real Madrid.
Kepa is second choice for Spain behind Manchester United's David de Gea.
He has spent the past two seasons in Athletic's first team, making 53 La Liga appearances, and has one international cap.
The fee Chelsea have paid surpasses the £60m they spent on Spanish striker Alvaro Morata in July 2017.
"So many things attracted me to the club - all the titles the club has won, the other players, the city, the English Premier League," Kepa said.
"It's an accumulation of things, and I am very glad Chelsea has decided to trust me and to take me in as well."
In January, Kepa signed a new long-term contract with Athletic until 2025, amid interest from Real Madrid.
He is the second key player to leave the club this year, after Manchester City signed French defender Aymeric Laporte for £57m in January.
"Kepa is a very solid goalkeeper, who can become an even better goalkeeper in the years to come," European football writer Andy Brassell told BBC Radio 5 live. "He was deemed good enough by the board and (former manager) Zinedine Zidane to be good enough for Real Madrid, but they wouldn't pay the clause."
'Calm, with lightning-fast reflexes' - analysis
Spanish football writer Andy West
It's ironic that Arrizabalaga is heading to Chelsea to replace Real Madrid-bound Courtois, because just a few months ago he nearly simplified the goalkeeper merry-go-round by moving to the Bernabeu himself.
In January, Real moved for Kepa by offering to pay his 20m euros buy-out clause with Athletic Bilbao, but he rejected that opportunity by signing a new seven-year contract with the Basque club.
As often happens, though, that only succeeded in raising his value rather than securing his long-term future, and Kepa is now the world's most expensive goalkeeper.
He joined Athletic at the age of nine. After advancing into the senior squad in 2012 his rise was steady rather than spectacular.
Following loan spells with second-tier Ponferradina and Real Valladolid, he finally became a starter for Athletic early in the 2016-17 season, quickly establishing himself by showing impressive poise and maturity.
Although Kepa conceded 43 goals in 30 league games last season, that was mainly due to the deficiencies of the weak team in front of him.
A more accurate reflection of his talent was his elevation into the Spain squad, although his outing in a 5-0 friendly win over Costa Rica in November 2017 remains his only international appearance to date.
His greatest assets are lightning-fast reflexes, allowing him to make stunning close-range stops, and calm composure in even the most frenetic of penalty areas.
And although eyebrows have been raised by the huge fee for a player with just two seasons of top-flight experience, the consensus in Spain regards Kepa as the country's best young goalkeeper with a very bright future.
A world record - for 20 days
Before Alisson moved from Roma to Liverpool in July, Gianluigi Buffon had spent 17 years as the most expensive goalkeeper in world football.
The 40-year-old Italian, who this summer left Juventus to join Paris St-Germain, was purchased for 53m euros by Juve from Parma on 3 July 2001.
While that record stood for 6,225 days, the fee paid for Alisson was surpassed within 20 days.