The 31-year-old joined Mikel Arteta’s side from Brighton back in January 2023 and went on to make 174 appearances across all competitions, scoring 36 goals.
Trossard became a key player in the Arsenal squad, coming up with vital goals, including the strike in their 1-0 win over West Ham last season.
Trossard has now completed his move to Turkish giants for a reported €18 million plus a potential €2 million in performance-based add-ons.
The Belgian forward has signed a three-year contract with Besiktas, which includes an option to extend for an additional 12 months.