Thae was the No. 2 diplomat in the North Korean embassy in London when he fled to South Korea with his wife and two sons in 2016.
South Korea's main conservative political opposition,the United Future Party,selected him to run in National Assembly's April elections.
Thae is the first North Korean defector to seek a popularly elected parliamentary seat in the South;another defector sat in Parliament from 2012 to 2016 as an expert nominated by a political party.
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On Wednesday,Thae said he had never thought of seeking election until the two North Korean fishermen were sent home against their will.
"I could not sleep that night," he said during a news conference.
South Korean navy commandos seized the two North Koreans from their squid-fishing boat in early November after they had repeatedly resisted an order to surrender,according to South Korean officials.