Sadiq al-Mahdi (Arabic: الصادق المهدي‎) (also known as Sadiq Al Siddiq, born December 25, 1935, Al-Abasya, Omdurman, Sudan) is a Sudanese political and religious figure. He is head of the National Umma Party and Imam of the Ansar, a sufi sect that pledges allegiance to Muhammad Ahmad who claimed to be Islam's messianic saviour, or the Mahdi.

Sadiq al-Mahdi was born on December 25, 1935 in Al-Abasya, Omdurman, Sudan. He is the grandson of Sayyid Abd al-Rahman al-Mahdi, founder of the Umma party, and great-grandson of Mohamed Ahmed Al-Mahdi, the Sudanese sufi sheikh of the Samaniyya order and self-proclaimed Mahdi who led the Mahdist War to liberate Sudan from its British occupiers. He is also the paternal uncle of Sudanese-British actor Alexander Siddig, who is best known for his role as Dr. Julian Bashir on Star Trek: Deep Space Nine.

Sadiq al-Mahdi was Prime Minister of Sudan on two occasions: first briefly in 1966-67, and second from 1986 until his ousting on 30 June 1989.

A controversy erupted in the late 1970s when he armed the Darfuri Arab tribes of Messiria and the Rizeigat as a part of an extraterritorial military incursion into Sudan from Libya. His uncle was killed during an air bombardment Nimeiry ordered on Aba Island.