Abū Isḥāq Muḥammad ibn al-Wāṯiq (died 21 June 870), better known by his regnal name al-Muhtadī bi-'llāh (Arabic: المهتدي بالله, "Guided by God") was the Abbasid Caliph in Baghdad from 869 to 870, during the "Anarchy at Samarra".
Al-Muhtadi's mother was Qurb, a Greek slave.
After the death of al-Mu'tazz, the Turks chose his cousin, al-Muhtadi as the new Caliph. Al-Muhtadi turned out to be firm and virtuous compared to the last few Caliphs. If he had come earlier, he might have restored life to the Caliphate; however, by now the Turks held more power.
Under him, the Court soon saw a transformation. Singing girls and musicians were expelled; justice was done daily in open court; wine and games were prohibited. He set Umar ibn Abd al-Aziz, the Umayyad Caliph, as his model and exemplar.
His reign, however, lasted less than a year. After some disagreements and conspiracies, he was killed by the Turks in 256 AH (June 870); he was thirty-eight then. The early Arab writers praise his justice and piety; and had he not been killed so soon, he could have been placed among the best of Abbasid Caliphs.