Al-Mansur or Abu Ja'far Abdallah ibn Muhammad al-Mansur (95 AH – 158 AH (714 AD – 775 AD); ) was the second Abbasid Caliph reigning from 136 AH to 158 AH (754 AD – 775 AD) and succeeding his brother Abu al-'Abbas al-Saffah. Al-Mansur is generally regarded as the real founder of the Abbasid Caliphate, one of the largest polities in world history, for his role in stabilizing and institutionalizing the dynasty. He is also known for founding the 'round city' of Madinat al-Salam which was to become the core of imperial Baghdad.

Abu Tahir Ismail al-Mansur Billah (913 – 19 March 953) was the third Caliph of the Fatimid Caliphate in Ifriqiya (r. 946-953).