Neven Subotic has left Borussia Dortmund to join Koln on loan until the end of the 2016-17 season.
The 28-year-old was poised to join Premier League side Middlesbrough in the close-season before a rib injury curtailed that move.
Subotic has yet to make a Bundesliga appearance for Dortmund this season, in part due to the injury, and his chances of first-team football at Signal Iduna Park appeared increasingly slim.
Consequently the centre-back has been allowed to leave Dortmund on a temporary basis.
"Bearing in mind what Neven has done for BVB, we decided to meet the player's request to be allowed to join Koln on loan until the end of the season in order to gain match rhythm at the highest level," sporting director Michael Zorc said.
Subotic joined Dortmund from Mainz in 2008 and has since made over 200 appearances in all competitions for the club, winning the Bundesliga title in 2011 and 2012.
"The way Neven identifies himself with Dortmund is quite rare in modern football," CEO Hans-Joachim Watzke added.
"Even though Neven will leave us on a temporary basis, we will keep him as a friend forever. Dortmund are and will always be his home. He is always welcome here."