Abu Abdallah Muhammad ibn Abdallah al-Mansur (Arabic: أبو عبد الله محمد بن عبد الله المنصور‎; 744 or 745 – 785), better known by his regnal name al-Mahdi bi-'llah (المهدي بالله, "He who is guided by God"), was the third Abbasid Caliph who reigned from 775 to his death in 785. He succeeded his father, al-Mansur.