The defector is being investigated after crossing into South Korea on Tuesday, the National Intelligence Service official said, without elaborating. He spoke anonymously because of agency policies.
Defections across the land border are rare. The last known defection across the border occurred in 2008 when a North Korean soldier crossed to the South.
The vast majority of North Koreans fleeing their communist homeland travel through China and Southeast Asia before arriving in the South.
More than 20,000 North Koreans have arrived in the South since the Korean War ended in a cease-fire, not a peace treaty, said South Korea's Unification Ministry.