U Thant (Burmese: ဦးသန္ ့့; January 22, 1909 – November 25, 1974) was a Burmese diplomat and the third Secretary General of the United Nations, from 1961 to 1971. He was chosen for the post when his predecessor, Dag Hammarskjöld, died in September 1961. "U" is an honorific in Burmese, roughly equal to Mr. "Thant" was his only name. In Burmese he was known as Pantanaw U Thant, in reference to his home town of Pantanaw.

Thant was born in Pantanaw, Lower Burma, and was educated at the National High School in Pantanaw and at University College, Rangoon, where he studied history. He was the eldest of four sons and was born into a family of well-to-do landowners and rice merchants. His father, Po Hnit, who came "from a mixed background, with both Muslim and Buddhist forebears", had helped establish The Sun (Thuriya) newspaper in Rangoon. He was also a founding member of the Burma Research Society. His father died when Thant was 14, and a series of inheritance disputes forced Thant's mother, Nan Thaung, and her four children into difficult financial times.

After university Thant returned to Pantanaw to teach at the National School and had become its headmaster by the age of 25. During this time he became close friends with future prime minister U Nu, who was from neighbouring Maubin and was the local superintendent of schools. Thant regularly contributed to several newspapers and magazines under the pen name "Thilawa" and translated a number of books, including one on the League of Nations. U Thant was a devout Buddhist.

When U Nu became the prime minister of the newly independent Burma, he asked Thant to join him in Rangoon and appointed him director of broadcasting in 1948. In the following year he was appointed secretary to the government of Burma in the Ministry of Information. From 1951 to 1957, Thant was secretary to the prime minister, writing speeches for U Nu, arranging his foreign travel, and meeting foreign visitors. During this entire period, he was U Nu's closest confidant and advisor.

He also took part in a number of international conferences and was the secretary of the first Asian - African summit in 1955 at Bandung, Indonesia, which gave birth to the Non-Aligned Movement. From 1957 to 1961, he was Burma's permanent representative to the United Nations and became actively involved in negotiations over Algerian independence. In 1961, the Burmese government awarded him the title Maha Thray Sithu as a commander in the order of Pyidaungsu Sithu.

Thant began serving as acting Secretary General from November 3, 1961, when he was unanimously appointed by the General Assembly, on the recommendation of the Security Council in Resolution 168, to fill the unexpired term of Dag Hammarskjöld. He was then unanimously appointed secretary general by the General Assembly on November 30, 1962, for a term of office ending on November 3, 1966. During this first term he was widely credited for his role in defusing the Cuban Missile Crisis and for ending the civil war in the Congo. He also said that he wanted to ease tensions between major powers while serving at the UN.

In April 1964, Thant accepted the Holy See’s designation of itself as a permanent observer. There appeared to be no involvement of the General Assembly or the UN Security Council in the decision. He received the Jawaharlal Nehru Award for International Understanding in 1965.

U Thant was re-appointed secretary general of the United Nations by the General Assembly on December 2, 1966, on the unanimous recommendation of the Security Council. His term of office continued until December 31, 1971, when he retired. During his time in office, he oversaw the entry into the UN of dozens of new Asian and African states and was a firm opponent of apartheid in South Africa. He also established many of the UN's development and environmental agencies, funds and programmes, including the UN Development Programme (UNDP), the UN University, UNCTAD, United Nations Institute for Training and Research (UNITAR), and the UN Environmental Programme.

Unlike his two predecessors, Thant retired after ten years on speaking terms with all the big powers. In 1961, when he was first appointed, the Soviet Union had tried to insist on a troika formula of three secretaries general, one representing each Cold War bloc, something which would have maintained equality in the United Nations between the superpowers. By 1966, when Thant was reappointed, all the big powers, in a unanimous vote of the Security Council, affirmed the importance of the secretary generalship and his good offices, a clear tribute to Thant's work.

The Six Day War between Arab countries and Israel, the Prague Spring and subsequent Soviet invasion of Czechoslovakia, and the Indo - Pakistani War of 1971 leading to the birth of Bangladesh all took place during his tenure as secretary general.

He was widely criticized in the US and Israel for agreeing to pull UN troops out of the Sinai in 1967 in response to a request from Egyptian president Nasser. U Thant tried to persuade Nasser not to go to war with Israel by flying to Cairo in a last minute peace effort.

His once good relationship with the US government deteriorated rapidly when he publicly criticized American conduct of the Vietnam War. His secret attempts at direct peace talks between Washington and Hanoi were eventually rejected by the Johnson Administration.

Thant followed UFO reports with some interest; in 1967, he arranged for American atmospheric physicist James E. McDonald to speak before the UN's Outer Space Affairs Group regarding UFOs.

On January 23, 1971, U Thant categorically announced that he would "under no circumstances" be available for a third term as secretary general. For many weeks, the UN Security Council was deadlocked over the search for a successor before finally settling on Kurt Waldheim to succeed U Thant as secretary general on December 21, 1971 — Waldheim's 53rd birthday — and just ten days before U Thant's second term was to end. In his farewell address to the United Nations General Assembly, Secretary General U Thant stated that he felt a "great sense of relief bordering on liberation" on relinquishing the "burdens of office". In an editorial published around December 27, 1971, praising U Thant, The New York Times stated that "the wise counsel of this dedicated man of peace will still be needed after his retirement". The editorial was titled "The Liberation of U Thant".

While serving as secretary-general, U Thant lived in Riverdale, Bronx, on a 4.75-acre (1.92 ha) estate near 232nd Street, between Palisade and Douglas avenues.

U Thant died of lung cancer in New York on November 25, 1974. By that time Burma was ruled by a military junta which refused him any honors. The then Burmese president Ne Win was envious of U Thant's international stature and the respect that was accorded him by the Burmese populace. Ne Win also resented U Thant's close links with the democratic government of U Nu which Ne Win had overthrown in a coup d'etat on March 2, 1962. Ne Win ordered that U Thant be buried without any official involvement or ceremony. From the United Nations headquarters in New York where he was laid in state, U Thant's body was flown back to Rangoon, but no guard of honour or high ranking officials were on hand at the airport when the coffin arrived except for U Aung Tun, deputy minister of education, who was subsequently dismissed from office. On the day of U Thant's funeral on December 5, 1974, tens of thousands of people lined the streets of Rangoon to pay their last respects to their distinguished countryman, whose coffin was displayed at Rangoon's Kyaikasan race course for a few hours before the scheduled burial. The coffin of U Thant was then snatched by a group of students just before it was scheduled to leave for burial in an ordinary Rangoon cemetery. The student demonstrators buried U Thant on the former grounds of the Rangoon University Students Union (RUSU), which Ne Win had dynamited and destroyed on July 8, 1962.

During the period of December 5–11, 1974, the student demonstrators also built a temporary mausoleum for U Thant on the grounds of the RUSU and gave anti-government speeches. In the early morning hours of December 11, 1974, government troops stormed the campus, killed some of the students guarding the makeshift mausoleum, removed U Thant's coffin, and reburied it at the foot of the Shwedagon Pagoda, where it has continued to lie. Upon hearing of the storming of the Rangoon University campus and the forcible removal of U Thant's coffin, many people rioted in the streets of Rangoon. Martial law was declared in Rangoon and the surrounding metropolitan areas. What has come to be known as the U Thant Crisis — the student led protests over the shabby treatment of U Thant by the Ne Win government — was crushed by the Burmese government.

In 1978, U Thant's memoirs, View from the UN, were posthumously published, initially by the American publishing house Doubleday.

U Thant had three brothers: Pantanaw U Khant, U Thaung, and U Tin Maung. U Thant was a licensed radio amateur, call-sign XZ2TH. He was married to Daw Thein Tin. U Thant lost both sons. Maung Bo died in infancy. Tin Maung Thant fell from a bus during a visit to Rangoon. Tin Maung Thant's funeral procession, which was attended by dignitaries, was grander than that of the state funeral of Commodore Than Pe, a member of the 17-man Revolutionary Council and minister of health and education. U Thant was survived by a daughter, an adopted son, five grandchildren, and five great-grandchildren (three girls and two boys). His only grandson, Thant Myint-U, is a historian and a former senior official in the UN's Department of Political Affairs and the author of The River of Lost Footsteps, in part a biography of U Thant.