South Korean soldiers guided the North Korean to safety after they found him walking across the eastern sector of the Demilitarized Zone, the buffer zone separating the Koreas, the South Korean military said in a brief statement.
South Korean and U.S. soldiers stand guard at the truce village of Panmunjom, July 2017.
A South Korean train is entering North Korea for the first time since 2007.
North Korea's leader Kim Jong-un shakes hands with US President Donald Trump at the start of their historic US-North Korea summit, at the Capella Hotel on Sentosa island in Singapore on June 12, 2018.
"I think it will be absolutely imperative in this next summit that we come away with a plan for identifying all of the weapons in question, identifying all the development sites, allowing for inspections of the sites and the plan for dismantling nuclear weapons", Pence said.
The last defection involving North Koreans occurred in May when two civilians aboard a small boat defected to the South across the Yellow Sea.
South Korea's President Moon Jae-in is determined that this railway project will go ahead, says our correspondent.More news: Will this Sikh shrine build corridors of peace between India and Pakistan?
South Korea said last week it had won exemptions for the joint survey. Three other soldiers reportedly crossed the land border past year in separate incidents.
In 2012, a North Korean soldier walked unchecked through rows of electrified fencing and surveillance cameras, prompting Seoul to sack three field commanders for the security lapse. "Through the one connected railway, the South and the North will prosper together and the ground for peace on the Korean Peninsula will be consolidated", he said.
A North Korean soldier fled across the heavily fortified frontier Saturday, the military said, a rare escape that presents the first test of recent military tension-reduction measures by the two adversaries.
Trump and Moon, meeting on the sidelines of the Group of 20 summit in Buenos Aires, "reaffirmed their commitment to achieve the final, fully verified denuclearization" of North Korea, Trump spokeswoman Sarah Sanders said.
North and South Korea have begun to remove landmines and destroy military bunkers at parts of their common border as part of efforts to improve long-strained relations.
They have also begun work to reconnect a train line and fix another rail link across the border.
The Koreas plan to hold a groundbreaking ceremony by the end of the year on an ambitious project to connect their railways and roads as agreed by their leaders. This reports the South Korean news agency "Yonhap", citing the army leadership in Seoul.