The 23-year-old Cologne native has put pen to paper on a new long-term deal until 2022. Horn has been first choice in goal for the Billy Goats since the 2012/13 season after rising through the youth ranks and has already made 155 first-team appearances for the club.
The shot-stopper has had an interrupted campaign so far as Cologne push for a place in Europe next year. He missed 13 games either side of the winter break after undergoing surgery on a knee injury
Although yet to break into the senior Germany team, Horn has experienced success on the international stage as he played in all six matches for his country at the 2016 Olympic Games in Rio de Janeiro, picking up a silver medal after a penalty shoot-out defeat to hosts Brazil in the final.