The incident happened at about 3:15 p.m. local time in South Korea (2:45 p.m. in North Korea) when the soldier drove a vehicle near the military demarcation line, which serves as the de-facto border between North and South Korea.
A spokesman for the South Korean military said the soldier was forced to abandon his car after a wheel came loose.
"He then exited the vehicle and continued fleeing south across the line as he was fired upon by other soldiers from North Korea," the UN Command, which is the unified command for U.S. and South Korean military forces, said in a statement.
The soldier, who had been struck by multiple bullets, took cover near a building on the southern side of the Joint Security Area, which is perhaps the border's best-known location as soldiers from South and North Korea stand face-to-face in the so-called Truce Village.
About 45 minutes later, the soldier was rescued by U.S. and South Korean forces and flown by helicopter to Ajou University Medical Center. "A UN Command security escort and translator accompanied the group," officials added.
The soldier - who was dressed in a uniform of the Korean People's Army but had no weapons with him - was seriously injured and remained unconscious on Tuesday. Doctors said the soldier suffered multiple gunshot wounds and will require a number of surgeries.
There was no immediate word on the soldier's identity or what may have motivated him to flee to South Korea. The Military Armistice Commission of the UN Command will investigate the circumstances of Monday's incident.