The German's contract, which expires at the end of the season, has not been renewed after Horn expressed a desire to be first-choice again, with Cologne having Marvin Schwabe as their main keeper since last season.
Horn joined the club as a nine-year-old in 2002 and made his first-team debut ten years later. He has 329 competitive games for the club.
"To be part of the team and achieve that makes me incredibly proud," Horn said in a statement. "However, my goal is to play every week and be there for my team."
Horn will be the second player to leave Cologne, as captain Jonas Hector will retire at the end of the campaign.