Abuʾl-Ḥārith Arslān al-Muẓaffar al-Basāsīrī (died 15 January 1059) was a Turkoman slave-soldier (mamlūk) who rose to become a military commander of the Buwayhid dynasty in Iraq. When the Buwayhids were ousted by the Seljuks in 1055, he transferred his allegiance to the Fatimid Caliphate of Egypt, in whose name he conquered Baghdad, which he ruled for almost a year.