While there are more than a thousand defections from North Korea to South Korea every year, most defectors come via China and it is rare for a North Korean to crossing the heavily mined Demilitarised Zone (DMZ). The last such crossing was in 2012.
The soldier approached a remote South Korean guard post in Gangwon province's Hwacheon county, in the central area of the peninsula, at about 8 a.m. on Monday, one Defence Ministry official said.
There was no exchange of fire or warning shots as the soldier clearly expressed his desire to defect, the official said. The soldier was being held in custody while officials ran checks, Reuters reported.
Stretching across the Korean peninsula, the DMZ is 4 kilometres (2.49 miles) wide and is fortified with landmines and barbed wire. There are telephones on the South Korean side for defectors from the North to call seeking help.
Yonhap News Agency reported that the North Korean military had been laying landmines along parts of the border this year, in an apparent move to prevent defections, citing a South Korean government official.
South Korean's defense ministry declined to confirm the report.