“It is difficult to leave Ajax but I think this is a good step for me now,” Klaassen told Everton’s website.
Everton earlier signed Jordan Pickford, 23, from Sunderland for £25m – a British record fee for a goalkeeper.
Klaassen joins Pickford as another new signing with manager Ronald Koeman determined to build on last season’s seventh place that brought them Europa League football.
The 2016 Dutch footballer of the year skippered the Ajax side that lost to Manchester United in last season’s Europa League final.
他成为科曼的优先签约人，因为对23岁的中场球员罗斯·巴克利（Ross Barkley）的未来存有疑问，他被告知如果他在古迪逊公园（Goodison Park）未能签下新合同，他将被出售。
Klaassen added: “The moment that I spoke with the manager directly I got a good feeling. He is an honest guy, we talked about the club, about the way he wants to play and that gave me a positive feeling.”
Koeman said: “He is a player who is only 24 but has a lot of experience and is a leader on the pitch. He has already captained Ajax for two seasons and that shows you the type of person he is.
“He is hardworking, likes to press and, of course, will give us more creativity and goals.”
The Toffees have also been strongly linked with a £16m move for AC Milan striker M’Baye Niang. The 22-year-old had a spell on loan in the Premier League at Watford last season.