"I'm looking forward to the challenge awaiting me in Düsseldorf," said the 61-year-old. "One of the most important tasks for the moment will be to continue to stabilise the team. Together with the fans we want to find a way to help the club experience more successful times once more."
Long search over
Köstner, who took temporary charge of Bundesliga side VfL Wolfsburg for nine games towards the end of 2012, has signed a contract until the end of the 2014/15 season.
The club parted ways with former tactician Mike Büskens at the end of November following a poor run of form that left the side just two points above the relegation zone in 15th place. Oliver Reck, who took over the reins on an interim basis, will resume his duties as goalkeeping coach.