Abū’l-Qāsim ʿAhmad al-Mustā‘lī b’il-Lāh (16 September 1074 – 12 December 1101) is the ninth Fatimid Caliph, and believed by the Mustaali Ismaili sect to be the 19th imam.

Al-Musta‘li is made caliph by Regent al-Afdal Shahanshah (1094–1121) as the successor to al-Mustansir.

By and large, al-Musta‘li is subordinate to Malik al-Afdal.

This leads to a power struggle within the Fatimids, and although the revolt of his brother Nizar is unsuccessful (ending with his death in prison), the break from the rules of succession cause a schism amongst the Ismaili Shia.

In Seljuk Syria and Persia, the Nizari sect will develo, one branch of which is known to history as the Hashshashin.

Supporters of Musta'li's imamate become known as the Mustaali sect.