A North Korean defected to South Korea on Friday after crossing the western sea border, the South's Joint Chiefs of Staff said.

The defector was rescued on Gyodong Island, near the Yellow Sea border, around 1 a.m., according to the JCS.

Questioning is under way to determine why and how the person fled the North, it added.

The investigation has yet to reveal how the man in his 20s crossed the sea border into the southern side. While monitoring the border area, a South Korean Marine spotted the defector and came to his rescue, according to the military.

This is the latest in a series of similar cross-border defection cases by North Korean citizens this year.

Last month, five North Koreans crossed the sea border, known as the Northern Limit Line, in the East Sea into the South in a small boat.

In June, a father and son crossed the East Sea border to take refuge in the South. (Yonhap)