The 22-year-old Belgian midfielder has joined Villa for approximately £50 million, making him their most expensive signing to date.
Villa is preparing for their Champions League return for the first time since 1983.
Onana, who joined Everton in 2022 for £33 million, made 72 appearances for the club and played a vital role in their efforts to avoid relegation.
Aston Villa expressed their excitement in an official statement: “Aston Villa is thrilled to announce the signing of Amadou Onana from Everton.”
Onana joins a list of summer arrivals including Ross Barkley, Ian Maatsen, Jaden Philogene, Samuel Iling-Junior, Cameron Archer, and Enzo Barrenechea as Emery builds a squad for Champions League competition.
Onana began his professional career with Hamburger SV in Germany, then moved to LOSC Lille in France’s Ligue 1 before his stint at Everton, where he made over 70 appearances and scored three goals in his final season.