The former Manchester United player had been expected to move to Monaco earlier in the week.
Shinji Kagawa has left Borussia Dortmund for a second time, joining Turkish giants Besiktas.
The Japanese international, who returned for a second spell with Dortmund from Manchester United in 2014, had been expected to leave Germany in the January window after hardly featuring for the Bundesliga leaders this season.
Having expressed an interest in moving to Spain two months ago, it seemed earlier in the week that his most likely destination would be AS Monaco, with the Ligue 1 strugglers reportedly poised to complete a temporary move for him.
Despite this, Kagawa arrived in Istanbul on Thursday to complete a loan move to Besiktas - a deal which could be made permanent in the summer.
The deal was announced by the Turkish club's official Twitter account on Thursday evening.
https://twitter.com/Besiktas/status/1091063597743173639
Besiktas currently sit sixth in the Turkish Super League, the division they last won the season before last.
Kagawa rose to prominence in his first spell with Dortmund, who he joined from Cerezo Osaka in 2010.
His form in Germany attracted the attention of Manchester United, with Sir Alex Ferguson signing him in 2012. Despite becoming the first Asian player to score a Premier League hat-trick, Kagawa failed to hold down a regular starting berth at Old Trafford and was sold back to Dortmund by Louis van Gaal.
