The contractor, Hanjin Heavy Industries, produces Seoul's naval vessels and amphibious assault vehicles.
After detecting the hack on 20 April, the nation's Defence Security Command began an investigation to determine if any "military secrets were leaked and whether North Korea was involved," unnamed officials told local broadcaster Yonhap.
It is suspected that North Korea hosts sophisticated hacking teams including one group dubbed Bureau 121 which may be a segment of the General Bureau of Reconnaissance within the nation's military.
Bureau 121 has been blamed for several cyber-attacks, including the 2014 hack of Sony Pictures which is thought to be retribution from Pyongyang over the studio's release of a film, The Interview, parodying the North Korean leader, Kim Jong-un.
North Korea said this latest claim was politically motivated and that the attacks were fabricated.