Onana agreed a five-year contract with Villa, who had been searching for a replacement for Douglas Luiz after the Brazilian's move to Juventus.
The 22-year-old failed to make an impact at Everton after arriving from Lille two years ago.
But, after featuring for Belgium at Euro 2024, Onana has been given the chance to revive his Premier League career by Villa boss Unai Emery.
"Aston Villa is delighted to announce the signing of Amadou Onana from Everton," the club said on their website.
Capped 17 times by Belgium, Onana made 72 appearances for Everton, scoring four goals.
He joins Villa with the club looking forward to their first Champions League campaign since 1983 after last season's fourth-place finish in the Premier League.
It has been a busy close season for Emery, who had already signed Ross Barkley from Luton, Juventus duo Samuel Iling-Junior and Enzo Barrenechea, Sheffield United's Cameron Archer, Ian Maatsen from Chelsea, former Hull winger Jaden Philogene and Everton's Lewis Dobbin.
Both Villa and Everton have been juggling their squads in a bid to stay within the Premier League's profit and sustainability rules.
