Jeremain Lens says his ambition to play at the 'highest level' was the reason behind joining Besiktas.
The Turkish giants won the 2016/17 Super Lig by four points and will compete in the Champions League this season. The move also comes as a significant blow to fierce rivals Fenerbahce, where Lens starred on loan last year. They had been keen to sign him this summer but concerns over their Financial Fair Play status prevented a deal.
Lens becomes Besiktas' latest high profile recruit, with Pepe and Alvaro Negredo also signing in recent weeks.
The Dutch winger said it was 'time to move on'.
"I'm really excited to come back to Turkey," he said on his Twitter page.
"I love Istanbul. What a great city. You gave me everything, I really felt that.
"Let me thank the fans, whether you're yellow and blue, or red and white.
"My ambition is to play at the highest level. It's time to move on."
Lens scored four goals in 24 appearances during two years on Wearside.
The deal should allow Simon Grayson to reinvest in his squad, with more attacking options and a centre-back key priorities.