Joseph Yun, who is in his early 60s, told CNN on Monday: "It was completely my decision to retire at this time."
He said US Secretary of State Rex Tillerson accepted his resignation "with regret".
Yun is leaving the State Department at a critical time, with expressions of interest in talks from both the US and North Korea in the wake of Pyongyang's participation in the Winter Olympics in South Korea.
Since his appointment as the US special representative for North Korea policy by former President Barack Obama's administration in October 2016, Yun has been a key player in implementing US strategy and dealing with bilateral relations, reports CNN.
He was instrumental in securing the release of Otto Warmbier, an American college student detained in North Korea for more than a year.
Yun and a medical team went to North Korea to pick him up last June.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
