The Korean will have a post on drug laws in Korea, but just know one thing -- marijuana is punished just as harshly as any other illegal drug in Korea. Also, many non-Koreans in Korea suffer through the unfair stereotype that they are all drug-addled criminals, because some visitors to Korea just cannot stop toking.
So please, do not smoke marijuana when you are in Korea. And if you must smoke, smoke in the privacy of your house or a hotel room, not in the smoking room of the Incheon Airport. And if you must smoke a joint in the smoking room of the Incheon Airport, please do not record it and put it up on Youtube, you fucking idiots.
인천공항 흡연실서 단체로 대마초 흡입? [Dong-A Ilbo]