A North Korean soldier killed two of his officers before crossing the heavily mined border into South Korea on Saturday, South Korea's defence ministry and media reports said.
Defections across the Demilitarized Zone, a buffer zone dividing the two Koreas, are rare as the 250 km-long (155 miles) land border is heavily armed and tightly guarded.
A defence ministry official confirmed a North Korean had defected across the land border, but provided no further details.
Local media quoted a statement from the Joint Chiefs of Staff as saying the North Korean soldier crossed the western section of the border at around noon.
The North Korean claimed that he shot dead his platoon and squad chiefs while on guard duty shortly before his border crossing, according to the reports.
The unnamed defector was being questioned by authorities.
The JCS statement was not available immediately.
Hundreds of North Koreans flee each year across its northern border with China and most make their way to the South, with more than 20,000 having found refuge in the wealthy capitalist neighbour.
Most cite economic hardship and political persecution as the main reasons for leaving home.
The two Koreas are still technically at war since the 1950-53 Korean War ended only with a ceasefire, not a peace treaty.