The K-League and Korea Anti-Doping Agency (KADA) made the decision at meeting of the disciplinary committee after the player failed a drug test on June 11 for the anabolic steroid methyltestosterone.
The 27-year-old, who had blamed a moustache growing cream for the failed test, apologised to fans and officials after attending the hearing, Korea's Yonhap News reported.
"We decided on a 15-game ban and a warning to the team with acknowledgement of the positive doping test based on the hearing report," Cho Nam-don, the head of the committee, was quoted as saying by Yonhap.
"It seems that the player didn't use that (the cream) to improve his performance."
Kang has already missed two games and so will sit out 13 more until the end of his suspension.
The Korean Football Association (KFA) said it would hold a separate meeting to decide whether he can play in the FA Cup.
Kang said he was sorry to let down his fans.
"This has been a very hard time for me," he said. "I am sorry to those who have supported me."
(Editing by Peter Rutherford)
