Jaʿfar ibn Muhammad al-Muʿtasim bi'llah (Arabic: جعفر بن محمد المعتصم بالله‎‎; March 822 – 11 December 861), better known by his regnal name al-Mutawakkil ʿala Allah (المتوكل على الله, "He who relies on God") was an Abbasid caliph who reigned in Samarra from 847 until 861. He succeeded his brother al-Wathiq. His assassination on 11 December 861 by the Turkish guard with the support of his son, al-Muntasir, began the troubled period of civil strife known as "Anarchy at Samarra".

When al-Wathiq died unexpectedly in August 847, the leading officials,the vizier Muhammad ibn al-Zayyat, the chief qādī, Ahmad ibn Abi Duwad, the Turkish generals Itakh and Wasif al-Turki, and a few others, assembled to determine his successor. Ibn al-Zayyat initially proposed al-Wathiq's son Muhammad (the future al-Muhtadi), but due to his youth he was passed over, and instead the council chose the 26-year-old Ja'far, who became the caliph al-Mutawakkil. Unbeknownst to them, the new Caliph was resolved to destroy the coterie of his father's officials that controlled the state, and rule on his own authority.

Al-Mutawakkil's first target was the vizier Ibn al-Zayyat, against whom he harboured a deep grudge over the way he had disrespected him in the past. According to al-Tabari, when al-Wathiq had grown angry and suspicious at his brother, al-Mutawakkil had visited the vizier in hopes of persuading him to intercede with the Caliph. Not only had Ibn al-Zayyat kept the Abbasid prince waiting until he finished going through his correspondence, but even mocked him, in the presence of others, for coming to him seeking assistance. Not only that, but when the dejected prince left, Ibn al-Zayyat wrote to the Caliph to complain about his appearance, noting that he was dressed in effeminate fashion, and that his hair was too long. As a result, al-Wathiq had his brother summoned to court. Al-Mutawakkil came in a brand-new court dress, hoping to mollify the Caliph, but instead al-Wathiq ordered that his hair be shorn off, and al-Mutawakkil be struck in the face with it. In later times, al-Mutawakkil confessed that he had never been so distressed by anything in his life than by this public humiliation. Thus, on 22 September 847, he sent Itakh to summon Ibn al-Zayyat as if for an audience. Instead, the vizier was brought to Itakh's residence, where he was placed under house arrest. His possessions were confiscated, and he was tortured to death.

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