(Arabic: “divinely guided one”), in Islāmic (Islām) eschatology, a messianic deliverer who will fill the Earth with justice and equity, restore true religion, and usher in a short golden age lasting seven, eight, or nine years before the end of the world. The Qurʾān (Islāmic sacred scriptures) does not mention him, and almost no reliable ḥadīth (saying attributed to the Prophet Muḥammad) concerning the mahdī can be adduced. Many orthodox Sunnī theologians accordingly question Mahdist beliefs, but such beliefs form a necessary part of Shīʿī doctrine.

The doctrine of the mahdī seems to have gained currency during the confusion and insecurity of the religious and political upheavals of early Islām (7th and 8th centuries). In 686, al-Mukhtār ibn Abū ʿUbayd at-Thaqafī (Mukhtār ibn Abū ʿUbayd al-Thaqafi, al-), leader of a revolt of non-Arab Muslims in Iraq, seems to have first used the doctrine by maintaining his allegiance to a son of ʿAlī (Muḥammad's son-in-law and fourth caliph), Muḥammad ibn al-Ḥanafīyah, even after al-Ḥanafīyah's death. Abū ʿUbayd taught that, as mahdī, al-Ḥanafīyah remained alive in his tomb in a state of occultation (ghaybah) and would reappear to vanquish his enemies. In 750 the ʿAbbāsid revolution made use of eschatological prophecies current at the time that the mahdī would rise in Khorāsān in the east, carrying a black banner.

Belief in the mahdī has tended to receive new emphasis in every time of crisis. Thus, after the battle of Las Navas de Tolosa (1212), when most of Spain was lost for Islām, Spanish Muslims circulated traditions ascribed to the Prophet foretelling a reconquest of Spain by the mahdī. During the Napoleonic invasion of Egypt, a person claiming to be the mahdī appeared briefly in Lower Egypt.

Because the mahdī is seen as a restorer of the political power and religious purity of Islām, the title has tended to be claimed by social revolutionaries in Islāmic society. North Africa in particular has seen a number of self-styled mahdīs, most important of these being ʿUbayd Allāh, founder of the Fāṭimid dynasty (909); Muḥammad ibn Tūmart, founder of the Almohad movement in Morocco in the 12th century; and Muḥammad Aḥmad, the mahdī of the Sudan who, in 1881, revolted against the Egyptian administration.