A South Korean soldier shot dead four members of his unit from a Marine Corps barracks on Ganghwa Island off the country's west coast, the defence ministry said.
Investigators are trying to determine why the marine, identified only by the Kim, opened fire on fellow soldiers, an army spokesman told reporters in Seoul.
The fourth wounded marine, who was taken to hospital, died of his injuries, the spokesman said.
The gunman sustained a minor injury, he added.
