Military personnel said suspect, identified only as Kim, was in custody after suffering a non-life threatening injury during the rampage in a barracks near the maritime border with North Korea, Yonhap news agency reported.
Marine spokesman Col. Kim Tae-eun said officials were still trying to determine what prompted the shooting and questioning service personnel in the barracks when the incident occurred.
"The marine corps has formed a task force and sent it to the military unit for an investigation," Kim said. "We sincerely apologize for having to inform people of this heartbreaking news."
Another official said the suspected shooter failed in a suicide attempt when he detonated a grenade, but the National Defense Ministry declined to comment on report, The Korea Times reported.
The colonel said Kim used a K-2 rifle, but the number of shots fired wasn't known, the South Korean news agency reported. Officials said a guard in the unit typically loads 15 bullets in the magazine.