Thant (Burmese: သန့်, MLCTS: san., pronounced [θa̰ɰ̃]; 22 January 1909 – 25 November 1974), known honorifically as U Thant (/uː θɑːnt/), was a Burmese diplomat and the third secretary-general of the United Nations from 1961 to 1971, the first non-Scandinavian as well as Asian to hold the position. He held the office for a record 10 years and one month.

A native of Pantanaw, Thant was educated at the National High School and at Rangoon University. In the days of tense political climate in Burma, he held moderate views positioning himself between fervent nationalists and British loyalists. He was a close friend of Burma's first Prime Minister U Nu and served in various positions in Nu's cabinet from 1948 to 1961. Thant had a calm and unassuming demeanour that won his colleagues' respect.

He succeeded James Barrington as Burma's Permanent Representative to the United Nations. He was appointed Secretary-General in 1961, six weeks after his predecessor, Dag Hammarskjöld, had died in an air crash. In his first term, Thant facilitated negotiations between U.S. president John F. Kennedy and Soviet premier Nikita Khrushchev during the Cuban Missile Crisis of 1962, helping to avert a global catastrophe. Later, in December that year, Thant ordered Operation Grandslam, which ended a secessionist insurgency in Congo. He was reappointed as Secretary-General on 2 December 1966, by a unanimous vote of the Security Council. During his second term Thant was well known for publicly criticizing U.S. conduct in the Vietnam War. He oversaw the entry of several newly independent African and Asian states into the UN. He refused to serve a third term, and retired in 1971.

Thant died of lung cancer in 1974. A devout Buddhist and the foremost Burmese diplomat on the international stage, he was widely admired and held in great respect by the Burmese populace. When the military government refused him any honours, riots erupted in Rangoon; these were violently crushed by the government, leaving scores of casualties.

Thant, the eldest of four sons, was born in Pantanaw, Colonial Burma, to a moderately wealthy family of landowners and rice merchants. His father Po Hnit, who had been educated in Calcutta, was the only person in the town who could communicate well in English. He was a founding member of the Burma Research Society and had helped establish The Sun (Thuriya) newspaper in Rangoon. Although his family members were ethnic Bamars and devout Buddhists, Thant's father, according to Thant Myint-U (U Thant's grandson), had distant ancestors who were "people from both India and China, Buddhists and Muslims, as well as Shans and Mons". He hoped that all his four sons would each earn a degree. His other sons, Khant, Thaung, and Tin Maung went on to become politicians and scholars.

Po Hnit had collected a personal library of various American and British books and cultivated a reading habit among his children. As a result, Thant became an avid reader and his school friends nicknamed him "The Philosopher". Apart from reading, he enjoyed various sports including hiking, swimming and playing chinlone. He went to the National High School in Pantanaw. At the age of eleven Thant participated in strikes against the University Act of 1920. He dreamed of becoming a journalist and surprised the family by writing an article for the Union of Burma Boy Scouts magazine. When Thant was fourteen, his father died and a series of inheritance disputes forced Thant's mother, Nan Thaung, and her four children into difficult financial times.

After the death of his father, Thant believed he would not be able to complete a four-year degree and instead worked for a two-year teaching certificate at Rangoon University in 1926. As the eldest son he had to fulfill his filial duties and responsibilities to the family. At university, Thant, together with future Prime Minister Nu studied history under D. G. E. Hall. Nu was told by a distant mutual relative to take care of Thant and the two soon became close friends. Thant was elected joint secretary of the University Philosophical Association and secretary of the Literary and Debating Society. In Rangoon, Thant met J.S. Furnivall, the founder of The Burma Book Club and The World of Books magazine, to which Thant regularly contributed. Promising a good post, Furnivall urged Thant to complete a four-year university course and join the Civil Service, but Thant refused. After earning the certificate, he returned to Pantanaw to teach at the National High School as a senior teacher in 1928. He contacted Furnivall and Nu regularly, writing articles and participating in The World of Books translation competitions.

In 1931, Thant won first place in All Burma Teachership Examination and became the school's headmaster by the age of twenty-five. Urged by Thant, his friend Nu took the local superintendent of schools position. 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. His major influences were Sir Stafford Cripps, Sun Yat-sen and Mahatma Gandhi. In the days of tense political climate in Burma, Thant stood moderate grounds between fervent nationalists and British loyalists.