Marshal Humberto de Alencar Castelo Branco (September 20, 1897 - July 18, 1967) was a Brazilian military leader and politician. He was President of Brazil, as a military dictator, after the 1964 coup d'etat. He left power in 1967 and was soon after killed when an aircraft he was travelling in was shot down by another Brazilian aircraft, allegedly by accident.

Castelo Branco was descended from a wealthy Northeastern family of overwhelming Portuguese ancestry (he can trace his ancestry back to the first King of Portugal Afonso Henriques). His physical appearance, according to Fabio Koifman, indicates Native American ancestry. His father, Cândido Borges Castelo Branco, had also been a general. His mother, Antonieta Alencar Castelo Branco, came from an intellectual family (which included the writer José de Alencar).

Castelo Branco married an Argentine woman, Viana, and had two children.

Castelo Branco joined the Brazilian Army in 1918. He was a student at a military school, the Escola Militar de Realengo (in Rio Grande do Sul), then in 1921 he joined the 12th Infantry Regiment in Belo Horizonte. In 1927, he returned to his military school as an infantry instructor. He was promoted to captain in 1938.

As a captain, he studied in France. He was made a lieutenant colonel in 1943. During World War II, he was a colonel in the Brazilian Expeditionary Force which fought in Italy against Germany. He served as "Chief of the Operations Section" ("chefe de seção de operações") and is said to have spent 300 days in combat zones.

Castelo Branco subsequently wrote a large number of academic studies and treatises on the conduct of war. He was appointed Chief of Staff of the Army by President João Goulart in 1963 and a marshal (of reserves) in 1964.

Castelo Branco became one of the leaders of the coup d'etat of March 31, 1964 that overthrew Goulart. On April 11, Congress chose him to serve out the balance of Goulart's term. He took the oath of office on April 15, 1964. Castelo Branco was the second Brazilian Field Marshal to become president of the nation through a coup d'état — the first was Deodoro da Fonseca, who deposed the Monarchic government of Emperor Dom Pedro II of Brazil in 1889.

Castelo Branco’s government, differently from previous directly elected presidents Juscelino Kubitschek, Jânio Quadros and João Goulart, was bankrolled from the start by the credits and loans from World Bank, IMF and massive investment from multinational American companies, which saw the Brazilian right wing military dictatorship as a new, economically stable Western ally against international Communism — mainly in Latin America — during the Cold War.

Castelo Branco was vested with emergency powers under the First Institutional Act, which among other things allowed him to cancel the political rights of "subversive elements" for 10 years. However, he was otherwise committed to permitting normal political activities while carrying out reform through legislation. He also had every intention of turning over power to a popularly elected president when Goulart's term was due to run out in 1966. However, several extreme right wing civilian and military elements felt the military needed to stay in power for a number of years in order to root out subversion. Events reached a breaking point in October 1965, when opposition candidates won the governorships of the major states of Minas Gerais and Guanabara. The extremists demanded that Castelo Branco annul the results, but Castelo Branco refused. A coup was only averted when War Minister Artur Da Costa e Silva persuaded the extremists to let the results stand in return for Castelo Branco's promise to take a tougher line.

Thereafter, Castelo Branco dropped all pretense of democracy. On October 27, he issued the Second Institutional Act, which abolished all existing political parties, restored his emergency powers, and extended his term to 1967. The numerous parties were replaced with only two: the pro-government "National Renewal Alliance Party" (ARENA) and the opposition "Brazilian Democratic Movement" (MDB). In 1967, he convened an extraordinary commission of jurists that crafted a highly authoritarian constitution. He issued many repressive laws, most notably a highly draconian press law (Lei de Imprensa) near the end of his term. This law continued to be valid in Brazil until 2009, when it was struck down by Brazil’s Supreme Federal Court. He was succeeded by Costa e Silva. He promoted government intervention into the economy (e.g. shutting down by decree the country's flag carrier, Panair do Brasil) and tributary reforms.

Six months after leaving the presidency, he died in a suspicious aircraft incident near Fortaleza. The aircraft in which he was flying is said to have been shot down accidentally by a "Shooting star" of the Brazilian Air Force.