Shinji Kagawa leaves Dortmund again, this time for Besiktas
Dortmund says Kagawa requested the move. The 29-year-old midfielder had only made two league appearances this season and did not feature prominently in coach Lucien Favre's plans.
Dortmund sporting director Michael Zorc says "Shinji is not only an outstanding football player, but also a great friend of the club. We couldn't refuse his request."
Kagawa was a key player in Dortmund's Bundesliga title wins in 2011 and 2012 following his move from Cerezo Osaka in 2010. He helped Dortmund achieve the double in 2012, scoring the first goal in the side's 5-2 win over Bayern Munich in the German Cup final.
Kagawa, who scored 60 goals and set up 55 more in 216 appearances for Dortmund, joined Manchester United in 2012 before returning in 2014, when he said the club was "like a family."
But his lack of opportunities at Dortmund this season prompted him to look elsewhere.
"We have a lot of understanding for Shinji's situation and for his desire for more playing time at another club," Zorc said. "Shinji sees this opportunity in Istanbul and we're happy to grant his wish."