Sadiq al-Mahdi, also known as Sadiq as-Siddiq, was a Sudanese political and religious figure who was Prime Minister of Sudan from 1966 to 1967 and again from 1986 to 1989. He was head of the National Umma Party and Imam of the Ansar, a Sufi order that pledges allegiance to Muhammad Ahmad (1844–1885), who claimed to be the Mahdi, the messianic saviour of Islam.

The Ansar are a Sufi religious movement in the Sudan whose followers are disciples of Muhammad Ahmad, a Sudanese religious leader based on Aba Island who proclaimed himself Mahdi on 29 June 1881. His followers won a series of victories against the Egyptians, culminating in the capture of Khartoum in January 1885.