Andrew Natsios, the US special envoy, resigned his post in late December in order to spend more time teaching, an unnamed US official told Reuters.
Natsios has been in the post since September 2006 and has worked towards finding a peaceful solution to civil war in the region of Darfur in the west of the country.
During his tenure, Natsios has also been teaching part-time at Georgetown University in Washington DC. He is quitting his post in order to return to the position on a full-time basis.
Several US officials have said that Natsios was frustrated at the slow pace of establishing a joint African Union-UN peacekeeping force in the region, said Reuters.
There have been widespread allegations that the introduction of the 26,000-strong force, which is set to begin operations in the new year, has been obstructed by the Khartoum-based national government.