Mohamed Siad Barre (Somali: Maxamed Siyaad Barre, Osmanya script: 𐒑𐒖𐒄𐒖𐒑𐒗𐒆 𐒈𐒘𐒕𐒛𐒆 𐒁𐒖𐒇𐒇𐒗; Arabic: محمد سياد بري;‎ c. 1910 – 2 January 1995) was a Somali head of state and general who served as the 3rd president of the Somali Democratic Republic from 1969 to 1991. He was given the childhood nickname Barre roughly referring to extraversion. Barre, a major general of the gendarmerie by profession, became President of Somalia after the 1969 coup d'état that overthrew the Somali Republic following the assassination of President Abdirashid Ali Shermarke. The Supreme Revolutionary Council military junta under Barre reconstituted Somalia as a one-party Marxist–Leninist communist state, renamed the country the Somali Democratic Republic and adopted scientific socialism (with support from the Soviet Union).

Barre's early rule was characterised by attempts at widespread modernization, nationalization of banks and industry, promotion of cooperative farms, a new writing system for the Somali language, and anti-tribalism. The Somali Revolutionary Socialist Party became Somalia's vanguard party in 1976, and Barre started the Ogaden War against Ethiopia on a platform of Somali nationalism and pan-Somalism. Barre's popularity was highest during the seven months between September 1977 and March 1978 when Barre captured virtually the entirety of the Somali region. It declined from the late-1970s following Somalia's defeat in the Ogaden War, triggering the Somali Rebellion and severing ties with the Soviet Union. Somalia then allied itself with the Western powers and especially the United States for the remainder of the Cold War, although it maintained its Marxist–Leninist regime and also drew close to China.

Opposition grew in the 1980s due to his increasingly dictatorial rule, growth of tribal politics, abuses of the National Security Service including the Isaaq genocide, and the sharp decline of Somalia's economy. In 1991, Barre’s government collapsed as the Somali Rebellion successfully ejected him from power, leading to the Somali Civil War and a massive power vacuum in its wake. Barre was forced into exile in Nigeria, where he died in 1995 on the way to the hospital after suffering a heart attack.