Abu Jafar ibn Muhammad al-Mansur (712-775) was the second caliph and real founder of the Arab Abbasid dynasty.

Abu Jafar, later al-Mansur, was the son of a Berber slave girl called Sallama and a brother of the first Abbasid caliph, Abu al-Abbas al Saffah. A great-great-grandson of Abd al-Abbas, the uncle of Mohammed, Abu Jafar and advocates of the Abbasid line considered themselves the true heirs of the Prophet as opposed to the Umayyads and the Shiite followers of Mohammed's cousin Ali. The rebellion of Abbasid forces in Khurasan in 747 led to defeat of the last Umayyad caliph, Marwan II, in 750 and the establishment of the Abbasid dynasty. During the fighting, Abu Jafar distinguished himself, particularly at the siege of Wasit.

Abu Jafar administered Armenia, Azerbaidzhan, and Mesopotamia during the short reign of Abu al-Abbas. Abu Jafar's succession as al-Mansur was announced during a pilgrimage to Mecca but...