Abu'l-Qasim Ahmad al-Mustansir (Arabic: أبو القاسم أحمد المستنصر; c. 1210 – 28 November 1261) was the first Abbasid caliph to rule in Cairo and who was subservient to the Mamluk Sultanate.
Abu'l-Qasim Ahmad was a member of the Abbasid house who was imprisoned by his nephew the Caliph al-Musta'sim in Baghdad.
Following the Sack of Baghdad by the Mongols in 1258, he escaped to the Arab tribes in the desert, where he hid out for over three years, until after the Mamluks had driven the Mongols from Syria in 1260.
He was sent with an army to the east to recover Baghdad, but was killed in a Mongol ambush near al-Anbar (near Falluja in modern Iraq) in 1261, and was succeeded, though not immediately, by his rather distant Abbasid kinsman (and former rival caliph, having been proclaimed by the ruler of Aleppo) Al-Hakim I.
Though he was not the direct ancestor of any of them, the line of Cairo caliphs Ahmad al-Mustansir founded lasted until the Ottoman conquest of Egypt in 1517, but they were little more than religious figureheads for the Mamluks.