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.
A spokesman for the Joint Chiefs of Staff said the soldier was in protective custody.
AFP, Reuters, TASS