Ryu Hyun Woo had led North Korea's embassy in Kuwait since former Ambassador So Chang Sik was expelled after a 2017 U.N. resolution sought to scale back the country's overseas diplomatic missions.
Ryu defected to South Korea last September, according to Tae Yong Ho, who was North Korea's deputy ambassador to Britain before settling in the South in 2016 and being elected as a lawmaker last year.
Kuwait had been a key source of foreign currency for Pyongyang, which sent thousands of laborers there, mostly for construction projects.
Tae said Ryu is also 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, dubbed Room 39.
The National Intelligence Service declined to comment.
Ryu fled several months after Jo Song Gil, who was North Korea's acting ambassador to Italy, vanished with his wife from the embassy and resurfaced in South Korea.
Tae told Reuters that the knowledge and experiences of the outside world gained as a diplomat had fostered disillusionment among his family, and he decided to escape to "give freedom" to his children, calling for other officials to follow suit.
"I want to deliver to my colleagues working around the world and North Korean elites that there is an alternative to North Korea, and the door is open," Tae said in an interview at the recent Reuters Next conference.