The 22-year-old played every minute of Belgium’s Euro 2024 campaign and now has the opportunity to represent Villa in the new season’s Champions League.
Onana joined Everton from Lille for £33m in 2022 and made 72 appearances for the Toffees, scoring four goals.
He was linked with a move to Arsenal in 2023, and a host of top clubs were reportedly keen this summer.
Villa boss Unai Emery had been in the market for a new defensive midfielder after the club sold Douglas Luiz to Juventus for £42.35m in June.