Abū al-ʿAbbās Aḥmad ibn al-Hasan al-Mustaḍīʾ, known as al-Nāṣir li-Dīn Allāh (1158–1225), was the Abbasid caliph in Baghdad from 1180 until his death, recognized for revitalizing the caliphate's influence and authority. Under his leadership, the Abbasid caliphate expanded its territory, notably conquering parts of Iran, marking him as the last effective Abbasid caliph according to historian Angelika Hartmann. Al-Nasir's reign saw the construction of significant monuments in Baghdad, including the Zumurrud Khatun Mosque and Mausoleum.

Al-Nasir's early reign was characterized by efforts to undermine Seljuq power, leading to the Seljuq Sultan of Persia, Toghrul III's defeat and death in 1194 at the hands of the Khwarezm Shah, Ala ad-Din Tekish, prompted by al-Nasir's instigation. This victory allowed Tekish to become the supreme ruler of the East and extend his dominion into previously Seljuq-controlled territories. Al-Nasir also engaged in reorganizing Baghdad's urban social groups, or futuwwa, aligning them with Sufi ideology to serve as an instrument of his governance.

Throughout his reign, al-Nasir encountered challenges and hostilities, particularly with the Khwarezm Shah, leading to periods of conflict and uneasy truces. Notably, his attempt to counter Tekish's son, Muhammad II, included controversial appeals to external powers, including possibly Genghis Khan, though this strategy ultimately exposed Baghdad to new threats. His reign was marked by significant military and political maneuvers, including alliances, conflicts, and diplomatic efforts across the Middle East.

Al-Nasir's rejection of Muhammad II's claim to shah in 1217 led to a failed invasion attempt by Muhammad towards Baghdad, thwarted by natural obstacles. The caliph's final years were plagued by illness, leading to his death in 1225, succeeded by his son al-Zahir. Despite a brief rule, al-Zahir's efforts to strengthen the caliphate were noted before his early death, succeeded by al-Nasir's grandson al-Mustansir.