Sithu U Nyun (20 January 1910 – 4 April 1996) was a Myanmar distinguished development economist, diplomat, public servant, and intellectual who served as the Executive Secretary as the head of United Nations Economic Commission for Asia and the Far East (ECAFE), which is now known as the United Nations Economic and Social Commission for Asia and the Pacific (ESCAP).

U Nyun was born on 20 January 1910 in Hpa-An, British Burma. He attended Basic Education High School No. 1 of Thaton and graduated from Rangoon University in 1930, and also attended the Oxford University and London Universities.

U Nyun joined the Indian Civil Service (ICS) on 4 October 1933. He was a senior civil servant in the independent Myanmar government, before joining the United Nations in 1953. From 1959 to 1973, he was the Under-Secretary General of the United Nations and the Executive Secretary (as the head) of ECAFE, working directly under United Nations Secretaries-General Dag Hammarskjöld and U Thant.

U Nyun was one of many trained and experienced Burmese ICS and BCS officers at the time of independence (U Ka Si, James Barrington, U Chan Tha, U Kyaw Min, U Kyaw Khine, U Balwant Singh, U Shwe Mra, U Win Pe, U Kyin, U Hla Shein etc). In the 1950s politics was messy and often violent; it was the work of these civil servants that played a big role in ensuring the country’s success.

During his career at ECAFE, he established the Mekong Development Project, Asian Highway Network, Asian Statistical Institute, Asian Industrial Development Council, Asian Development Bank, and Asian Clearing Union. In 1966, he founded the Asian Development Bank, and was recognized as The Father of Asian Development Bank. In 1973, he retired from the United Nations.

In 1935, he married to Daw Than Tin. He died on 8 April 1996 in Yangon, Myanmar. He is survived by his four sons and one daughter– U Aung Nyun, U Than Nyun, U Myo Nyun, U Hla Nyun and Daw Marie Nyun.