The North Korean soldier was spotted moving towards South Korea, and then crossed over a military demarcation line separating the two sides, the South's Joint Chiefs of Staff said in a statement.
A North Korean soldier fled across a heavily fortified border to defect to South Korea early Saturday, the military in Seoul said.
South Korean soldiers escorted the defector to safety after finding him moving south of the eastern side of the military demarcation line that bisects the Koreas, South Korea's Joint Chiefs of Staff said in a statement.
South Korean authorities plan to question the defector over the details of his escape. The Joint Chiefs of Staff said it had not observed any unusual activity from North Korean troops in the area where the defection happened.
It comes as the North and South Korea have pushed to implement a wide-ranging military agreement reached in September to reduce tensions across their border.
South Korea's Defence Ministry said on Friday that the Korean militaries completed removing 20 front-line guard posts and landmines from a border area where they plan to start their first-ever joint search for remains of soldiers killed during the 1950-53 Korean War.
The Koreas and the US-led UN Command recently finished removing firearms and troops from a jointly controlled area at the border village of Panmunjom, and eventually plan to allow tourists to freely move around there.
Last November, a North Korean soldier was critically wounded in the jointly controlled area after he fled to the South amid a hail of bullets fired by his former comrades.