The man identified only by his surname, Lee, was detained for selling sensitive data about the South’s military to a foreign agent, Seoul Central District Prosecutors' Office said, according to Yonhap.
He is suspected of leaking 109 pieces of data, including personal details of South Korean spies working abroad, to a person from an East Asian diplomatic mission, which was not revealed.
The classified military information was reportedly obtained from former ranking officials at the South Korea Defense Intelligence Command with the Ministry of National Defense.
South Korea's National Intelligence Service said earlier that North Korea had stolen information from dozens of Seoul top government officials' smartphones.