Abū'l-ʿAbbās ʿAbdallāh al-Ma'mūn ibn Hārūn al-Rashīd (; September 786 – 9 August 833) was the seventh Abbasid caliph, who reigned from 813 until his death in 833. He succeeded his brother al-Amin after a civil war, and presided over the greatest cultural and scientific flowering of Baghdad, but also over the Mu'tazilism controversy, and the resumption of large-scale warfare with the Byzantine Empire.