"He crossed the military demarcation line (MDL) in the middle eastern sector at around 10 a.m. and defected to the South," the Joint Chiefs of Staff said in a statement, as quoted by the Yonhap news agency.
The soldier is currently being questioned by the South on the reason behind his defection and how he managed to escape from the Hermit Kingdom.
The demarcation line runs along the middle of the 4-kilometer (2.5-mile) demilitarized zone on the border between the two countries.
Since 2010, around 60 people fled from Pyongyang’s rule across the demarcation zone. The last time a soldier crossed from the North to the South was in June 2015.