Ismaïl Omar Guelleh (born 27 November 1947), often referred to by his initials IOG, is a Djiboutian politician and dictator who has served as the President of Djibouti since 1999.

Guelleh was first elected as President in 1999 as the handpicked successor to his uncle, Hassan Gouled Aptidon, who had ruled Djibouti since independence in 1977. Guelleh was re-elected in 2005, 2011, 2016 and in 2021. The elections were largely boycotted by the opposition amid complaints over widespread irregularities. Guelleh has been characterized as a dictator, and his rule has been criticized by human rights groups and foreign governments.

Guelleh was born in Dire Dawa, Ethiopia, into the politically powerful Mamassan subclan of the Ciise Dir clan of the Somali ethnic group. The father of Ismaïl Omar Guelleh, son of Guelleh Batal, is Omar Guelleh, one of the first native teachers in the 1930s before working, following his father's path, on behalf of the Franco-Ethiopian Railway Company (CFE) which built the line connecting Djibouti to Addis Ababa and whose head office was in Dire Dawa. When Guelleh was younger he attended a traditional Islamic school. In 1960, Guelleh migrated to Djibouti, then known as French Somaliland, before finishing high school. In 1964, at the age of 18, Guelleh began working in the French territory's general information service, because he spoke Amharic, Somali, Arabic, French, Italian and English.

In 1975, he was suspended from his duties because he was suspected of transmitting information to the independence movement. He then became involved in the African People's League for Independence (LPAI) chaired by Hassan Gouled Aptidon, who campaigns for independence. Guelleh was elected into the LPAI's Central Committee in 1983 and subsequently became the director of a cultural commission in Paris. In 1987, he became a member of the party leadership.

After Djibouti became independent, he became head of the secret police and chief of the cabinet in the government of his uncle Hassan Gouled Aptidon, for whom he also served as chief of staff for more than two decades. He received training from the Somali National Security Service and then from the French Secret Service, and was intended to become his uncle's successor.

In February 1991 he attempted to annex Zeyla in Awdal, Somaliland to Djibouti in the 1991 Zeila incursion during the Somaliland War of Independence, however the Djiboutian-backed United Somali Front was shortly routed from the area by Somali National Movement (SNM) forces.

He became President of Djibouti in 1999.

Djibouti is one of four countries in Africa that have experienced instances of immediate successions from one family member to another.