Abu Ja'far Harun ibn Muhammad al-Mu'tasim (Arabic: أبو جعفر هارون بن محمد المعتصم‎; 816 – 10 August 847), better known by his regnal name al-Wathiq bi'llah (واثق بالل, "He who trusts in God"), was an Abbasid caliph who reigned from 842 until 847 AD (227–232 AH in the Islamic calendar).