Muhammad Ahmad bin Abd Allah (Arabic: محمد أحمد ابن عبد الله; 12 August 1844 – 22 June 1885) was a Sufi religious leader of the Samaniyya order in Sudan who, as a youth, studied Sunni Islam. In 1881, he claimed to be the Mahdi. He led a successful war against Ottoman-Egyptian military rule in Sudan and achieved a remarkable victory over the British, in the Siege of Khartoum. He created a vast Islamic state extending from the Red Sea to Central Africa, and founded a movement that remained influential in Sudan a century later.From his announcement of the Mahdiyya in June 1881 until 1898, the Mahdi's growing number of supporters, the Ansars, established many of its theological and political doctrines. After Muhammad Ahmad's unexpected death on 22 June 1885, his chief deputy, Abdallahi ibn Muhammad took over the administration of the nascent Mahdist state.

Following Ahmad's death, Abdallahi ruled as Khalifa but his autocratic rule, as well as directly applied British military force, destroyed the Mahdi state following the Anglo-Egyptian conquest of Sudan in 1899. Despite that, the Mahdi remains a respected figure in the history of Sudan. In the late 20th century, one of his direct descendants, Sadiq al-Mahdi, twice served as prime minister of Sudan (1966–67 and 1986–89). He pursued democratizing policies.