Abu Ali al-Mansur ibn al-Musta'li, better known by his regnal name al-Amir bi-Ahkam Allah was the tenth Fatimid caliph, ruling from 1101 to his death in 1130, and the 20th imam of the Musta'li Isma'ili branch of Shia Islam.

The Musta‘lī are a branch of Isma'ilism named for their acceptance of al-Musta'li as the legitimate nineteenth Fatimid caliph and legitimate successor to his father, al-Mustansir Billah. In contrast, the Nizari—the other living branch of Ismailism, presently led by Aga Khan IV—believe the nineteenth caliph was al-Musta'li's elder brother, Nizar. Isma'ilism is a branch of Shia Islam.