North African Islamic theologian who founded a militant mystical movement, the Sanūsīyah, which helped Libya win its independence in the 20th century.

During his formative years in his native country, which was incorporated in the Ottoman Empire, as-Sanūsī observed the corruption of the Ottoman administrators. To continue his religious studies, in 1821 he went to Fès, in Morocco. Morocco was then nominally independent but actually a colony of France. As-Sanūsī's experiences under foreign rule and his observation of the inherent weakness of the Islamic states convinced him of the need for a revitalized Islamic community.

After a pilgrimage to Mecca in 1828, as-Sanūsī visited Egypt. Attracted to mysticism in Morocco, in Egypt he joined many different orders, apparently in the hope of discovering as much as possible about their organization and doctrines. In 1837, while in the Hejaz (now in Saudi Arabia), he founded his own order, which later became known as the Sanūsīyah. He limited his activities to the Bedouin tribes of the area, and he made no effort to challenge tribal patterns of authority or religious beliefs. In 1841 he was expelled from the Hejaz by the Ottoman authorities, and in 1843 he moved the order to Cyrenaica, where he used the same tribal tactics as before to create a reliable instrument for challenging the existing power structure. In c. 1856 the order was moved to Jaghbūb, also in Cyrenaica but away from the sphere of Egyptian and Ottoman political control and near the caravan route from North Africa to the Hejaz and equatorial Africa. This location offered an excellent seat from which propaganda work could be carried out among the tribes.

The Sanūsīyah became popular among the tribes of Cyrenaica. In the 20th century it spearheaded the liberation movement against Italian colonization. As-Sanūsī's grandson Idrīs I ruled as king of Libya from 1951 to 1969.