The Red Devils disclosed this ahead of their Premier League derby with Manchester City on Sunday and wished Onana success at his new club.
Onana will spend the 2025/26 season with the Turkish side, having made 102 appearances for United.
The Cameroonian international joined Old Trafford from Italian Serie A giants Inter Milan in July 2023.
The 29-year-old was part of the United squad that lifted the Emirates FA Cup after a 2-1 victory over City at Wembley Stadium.
