Ryu Hyun-woo was acting ambassador to Kuwait and is thought to have fled with his family in September 2019, the Maeil business daily said.
About 30,000 North Koreans have fled repression and poverty under the communist regime and settled in the capitalist South, mostly by first secretly crossing over the porous border with China.
He was serving as a charge d'affaires after a North Korean ambassador was expelled from Kuwait following a 2017 U.N. Security Council resolution over North Korea's missile activities, according to the media.
Mr Ryu is the son-in-law of Jon Il Chun, who once oversaw a Worker's Party bureau responsible for managing the ruling Kim family's secret coffers, Mr Tae explained.
Ryu Hyun-woo, who had served as the embassy's charge d'affaires since Ambassador So Chang-sik was expelled from Kuwait after a United Nations resolution was adopted in 2017, entered South Korea, along with his family, according to the source.
Ryu reportedly entered South Korea in September 2019, about two months after former North Korean ambassador to Italy Jo Song-gil is known to have entered South Korea.
"I chose to defect because I wanted to offer my child a better future", the Maeil Business newspaper cited the former diplomat as saying.
The North has tightened border security as part of its defenses against the coronavirus, and the number of arrivals to the South plummeted previous year.
North Korea, which proclaims itself as a socialist paradise, is extremely sensitive about defections, especially among its elite, and has sometimes maintained that they are South Korean or American plots to undermine its government.