The unidentified reservist opened fire with his rifle during shooting practice at the camp in southern Seoul, a ministry spokesman said.
"The man killed himself on the spot following the shooting spree that killed one of his fellow reservists and wounded three others," the spokesman told AFP.
"The exact reason is still not clear, but our initial investigation found the incident appeared to be related to a personal matter," he added.
Upon completion, they must serve in the reserve forces for eight more years, with a maximum of 160 hours of duty per year.
While there have been a number of similar shooting incidents involving conscripts doing their full-time service, such episodes are relatively rare among part-time reservists.