Al-Musta'in (836 – 17 October 866) is the Abbasid Caliph in Baghdad from 862 to 866, during the "Anarchy at Samarra".

After the death of previous Caliph, al-Muntasir, (the previous Caliph had appointed no successors) the Turkish chiefs hold a council to select his successor; they will have none of al-Mu'tazz, nor his brothers; so they elect him, another grandson of al-Mu'tasim.