The 19-year-old England U20 international joins on a four-and-a-half-year deal until June 2021.
"Everton has a big history and I was also attracted by the manager," Lookman added.
"It was great playing in the Championship last season and for the last six months in League One but I feel like I'm ready to make the step up to the Premier League."
Toffees boss Koeman said: "Ademola is a big talent and, at 19 years old, he has a big future in the game. I'm really happy that we've been able to bring him here to the club."