A former North Korean diplomat has won a parliamentary seat in South Korea's swankiest district, four years after fleeing a London embassy and defecting to the South.

Thae Yong Ho was Pyongyang's deputy ambassador to the United Kingdom where he had managed secret funds for leader Kim Jong Un until he fled to the democratic, wealthy South in 2016.

Thae, 55, won one of three constituencies in Seoul's glittering Gangnam district for the main opposition conservative party in Wednesday's parliamentary election, beating a former four-term ruling party lawmaker by nearly a 20 percent margin.