The defector was critically wounded, having been hit at least four times in the hail of bullets as he raced for the border in a jeep and then on foot, where he collapsed and was dragged to safety by South Korean soldiers.
Doctors announced he had regained consciousness after two operations to extract the bullets.
"He is fine," lead surgeon Lee Cook-Jong told a press conference. "He is not going to die."
North Korean's actions during the defection violated the 1953 armistice agreement marking the cessation of hostilities in the Korean War, a spokesman for the UN command said.
Chad Carroll said a soldier from the North Korean People's Army (KPA) had crossed the Military Demarcation line (MDL), the border between the two Koreas, for a few seconds as others fired shots at the defecting soldier.
The defection, subsequent surgeries and slow recovery of the soldier have riveted South Korea, but it will be a huge embarrassment for the North, which claims all defections are the result of rival Seoul kidnapping or enticing North Koreans to defect. Pyongyang has said nothing about the defection so far.
The video shows the soldier speeding down a tree-lined road in a 4x4 military jeep, past shocked North Korean soldiers, who begin to run after him.
He crashes the jeep into a ditch near the line dividing North and South and the blue huts familiar to anyone who's toured the area, which is the part of the border where soldiers from the opposing countries face each other at their closest distance just metres apart. There were no tour groups at the time of the defection, Mr Carroll said.
Soldiers from the North sprint to the area, firing handguns and AK rifles — about 40 rounds, the South says — at the defector. One soldier hurries across the dividing line before running back to the northern side.
South Korean soldiers then crawl up to the defector, who has fallen injured in a mass of leaves against a small wall. They drag him to safety as North Korean troops begin to gather on their side of the line.
While on average more than 1,000 North Koreans defect to the South every year, most travel via China and it is unusual for a North Korean to cross the land border dividing the two Koreas, which have been in a technical state of war since their 1950-53 conflict ended in a truce rather than a peace treaty.
While treating the soldier's wounds, surgeons removed dozens of parasites from his ruptured small intestine, including presumed roundworms as long as 27cm (10.6 inches), which may reflect poor nutrition and health in North Korea's military.
The soldier is 1.7m (5 feet, 7 inches) tall, but weighs just 60kg (132 pounds).
Additional reporting by agencies
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