Report says, the Netherlands international striker, Klaasen has signed a five-year contract for a fee of £23.6m, which follows hot on the heels of their deal to land goalkeeper Jordan Pickford from Sunderland.
A 24-year-old Klaasen said: "I spoke a few weeks ago with Ronald Koeman about Everton," “I wanted to come here and check if this was the right club for me and, since I came here, I feel so happy. It's a great club.
"It's difficult to leave Ajax but I think this is a good step for me now and I'm really looking forward to my time here.
"I'm an attacking midfielder who likes to get in the box. I also like to be aggressive on the field and press all the time. I like to score and create goals and I like to win."
Toffees boss Ronald Koeman described his countryman as "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," Koeman said. "He is hardworking, likes to press and, of course, will give us more creativity and goals."