, Abū Isḥāḳ Muḥammad b. Hārūn al-Ras̲h̲īd , ʿAbbāsid caliph, reigned 218-27/833-42, son of the caliph Hārūn by a slave concubine Mārida.

During the reign of his brother and predecessor al-Maʾmūn [q.v.], al-Muʿtaṣim achieved a reputation as a skilful commander in Anatolia and as governor in Egypt. When al-Maʾmūn died in the Byzantine marches in Rad̲j̲ab 218/August 833, al-Muʿtaṣim was recognised as caliph despite support within the army for his nephew al-ʿAbbās b. al-Maʾmūn (who was, in fact, later to conspire against al-Muʿta…