Mr Hwang defected after witnessing North Korea's famine
South Korea says it has uncovered a plot to assassinate the most senior official ever to have defected from Communist North Korea.
Two North Koreans, said to have been posing as defectors themselves, have been arrested on suspicion of being on a mission to kill Hwang Jang-yop.
Mr Hwang, 87, once a close confidant of North Korean leader Kim Jong-il, defected to the South in 1997.
Pyongyang's official government website had recently threatened him with death.
The alleged plot to kill Mr Hwang was uncovered when the two men, named by the Yonhap news agency as Kim and Tong, crossed into South Korea from Thailand earlier this year, posing as defectors themselves.
They were questioned by South Korean officials during the debriefing sessions that await all North Korean refugees who make it to Seoul.
A unnamed senior official at Seoul Central District Prosecutors' Office told reporters they had said their orders were to "slit the betrayer's throat", the Associated Press news agency reports.
Mr Hwang, who was once the secretary of the North Korean Workers' Party, has said he left the country after witnessing the impact of disastrous economic policies which led to widespread famine in the 1990s.
He left close family members behind, many of whom are reported to have been sent to labour camps.
Mr Hwang lives under heavy police protection at an undisclosed location and has remained a harsh critic of Pyongyang.
He recently travelled to the United States to give a lecture, telling journalists that he has no regrets about his actions.
The BBC's John Sudworth in Seoul says it appears North Korea has never forgotten Mr Hwang's betrayal.
Just two weeks ago its official government website threatened him with death and described him as a "traitor and human scum".
But Mr Hwang was reported to have shrugged of the alleged threat when told of it on Wednesday.
"Why would you be alarmed by something like that?" the Yonhap news agency quoted him as saying.