Mahdi [Arab.,=he who is divinely guided], in Sunni Islam, the restorer of the faith. He will appear at the end of time to restore justice on earth and establish universal Islam. The Mahdi will be preceded by al-Dajjal, a Muslim antichrist, who will be slain by Jesus. This belief is not rooted in the Qur'an but has its origins in Jewish ideas about the Messiah and in the Christian belief of the second coming of Jesus. Among the Shiites the concept of the Mahdi takes a different form (see imam).

In the history of Islam, many men have arisen who claimed to be the Mahdi. They usually appeared as reformers antagonistic to established authority. The best known of these in the West was Muhammad Ahmad, 1844-85, a Muslim religious leader in the Anglo-Egyptian Sudan. He declared himself in 1881 to be the Mahdi and led a war of liberation from the oppressive Egyptian military occupation. He died soon after capturing Khartoum. In his reform of Islam the Mahdi forbade the pilgrimage to Mecca and substituted the obligation to serve in the holy war against unbelievers. His followers, known as Mahdists, for a time made pilgrimages to his tomb at Omdurman. The final defeat of the Mahdists in 1898 at Omdurman by an Anglo-Egyptian army under Lord Kitchener gave Great Britain control of Sudan.