The flag of Tanzania consists of a Gold-edged black diagonal band, divided diagonally from the lower hoist-side corner, with a green upper triangle and light blue lower triangle. Adopted in 1964 to replace the individual flags of Tanganyika and Zanzibar, it has been the flag of the United Republic of Tanzania since the two states merged that year. The design of the present flag incorporates the elements from the two former flags. It is one of a relatively small number of national flags incorporating a diagonal line, with other examples including the DR Congo, Republic of the Congo, Namibia, Trinidad and Tobago and Brunei.

Tanganyika was a sovereign state, comprising the mainland part of present-day Tanzania, that existed from 1961 until 1964. It first gained independence from the United Kingdom on 9 December 1961 as a Commonwealth realm headed by Queen Elizabeth II before becoming a republic within the Commonwealth of Nations a year later. After signing the Articles of Union on 22 April 1964 and passing an Act of Union on 25 April, Tanganyika officially joined with the People's Republic of Zanzibar to form the United Republic of Tanganyika and Zanzibar on Union Day, 26 April 1964. The new state changed its name to the United Republic of Tanzania within a year.