He suffered gunshot wounds to his elbow and shoulder and was taken to the hospital when South Korean soldiers found him about 25 minutes later on the southern side of the Joint Security Area, a strip of land where North and South Korean forces stand face-to-face, the military said, according to the South's Yonhap News Agency.
A North Korean soldier fled to South Korea through a hail of bullets in the Joint Security Area on Monday afternoon.
He was shot by a North Korean soldier but succeeded in crossing, the JCS added, and has been evacuated for medical treatment. Lee Guk-jong, a doctor who leads Ajou's medical team for the soldier, described his patient's condition as "very dangerous" and said the next 10 days might determine whether he recovers. This year, North Korean defectors successfully escaping the regime fell by 12.7 percent, according to the Telegraph.
The United States and South Korea on Saturday started joint naval exercises that will involve three US aircraft carriers in what military officials describe as a clear warning to North Korea.
Oceans Minister Kim Young-choon on Tuesday interpreted North Korea's recent repatriation of a seized South Korean fishing boat as a "conciliatory gesture", as Seoul seeks to improve cross-border ties. The last time a soldier crossed was in 2007, and before that in 1998.
The leaders of South Korea and China on Saturday reaffirmed their joint efforts to peacefully resolve North Korea's nuclear problem in a summit highlighting the recent normalization of bilateral ties estranged by the deployment of a US missile defense system in South Korea.
South Korean officials have yet to identify where the soldier came from or what his intentions were.
The U.N. Command, in place since the end of the war, said an investigation into the incident was being conducted. More than a million mines are buried inside the zone.
The fall is believed to be a result of tighter government surveillance and reinforced border security by both North Korea and China.
The North's official media hasn't reported about the case, but it previously accused of Seoul of kidnapping or enticing its citizens to defect.
The 1984 exchange of gunfire happened after North Korean soldiers crossed the border and fired, according to the U.N. Command.
Xi noted his meeting with the South Korean leader marked a turnaround in South Korea-China relations.
The defection came at a time of heightened tension over North Korea's nuclear weapons programme, and could escalate animosities between the rival countries.
The Associated Press contribute to this report.