FILE - Ivorian defender Eric Bailly vies for the ball during Marseille in the French League One football match between Marseille and Paris Saint-Germain at the Velodrome stadium in Marseille, southern France, on Feb. 26, 2023. Photo: AP
Besiktas specified neither the length of the contract nor the value of the transfer.
Having arrived in Manchester from Villarreal in the summer of 2016 and played a full first season, Bailly gradually slipped down the pecking order of United defenders.
Bailly, 29, was loaned to Marseille last season with an option to buy but his playing time was limited by a number of injuries.
Besiktas said in a statement: "We wish Eric Bailly, who we believe will provide important services to our club, great success with our glorious jersey, and present it to the public with our respect."
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