Abū Isḥāq Muḥammad ibn Hārūn al-Rashīd (Arabic: أبو إسحاق محمد بن هارون الرشيد‎‎; October 796 – 5 January 842), better known by his regnal name al-Muʿtaṣim bi’llāh (المعتصم بالله, "he who seeks refuge in God"), was the eighth Abbasid caliph, ruling from 833 to his death in 842. A younger son of Harun al-Rashid, he rose to prominence through his formation of a private army composed predominantly of Turkish slave-soldiers (ghilmān or mamālīk). This proved useful to his half-brother, Caliph al-Ma'mun, who made use of al-Mu'tasim and his Turkish guard to counterbalance other powerful interest groups in the state, as well as employing them in campaigns against rebels and the Byzantine Empire. When al-Ma'mun died unexpectedly in campaign in August 833, al-Mu'tasim was thus well placed to succeed him, overriding the claims of his nephew, al-Abbas ibn al-Ma'mun.

Al-Mu'tasim continued many of his brother's policies, like the partnership with the Tahirids and the support for Mu'tazilism and the miḥna, backed by the powerful chief qādī, Ahmad ibn Abi Duwad. Despite his own disinterest in literary pursuits, al-Mu'tasim also nurtured the scientific renaissance begun under al-Ma'mun. In other ways, however, his reign marks a departure and a watershed moment in Islamic history, with the creation of a new regime centred on the military, and particularly his Turkish guard. In 836, a new capital was established at Samarra to symbolize this new regime and remove it from the populace of Baghdad. The power of the caliphal government was increased by centralizing measures that reduced the power of provincial governors in favour of a small group of senior civil and military officials in Samarra, and the fiscal apparatus of the state was more and more dedicated to the maintenance of the professional army, which was dominated by the Turks. An abortive conspiracy against al-Mu'tasim in favour of al-Abbas in 838 resulted in a widespread purge of the previous Arab and Iranian elites that had played a major role in the early period of the Abbasid state, and the strengthening of the position of the Turks and their principal leaders, Ashinas, Wasif, Itakh, and Bugha. Another prominent member of al-Mu'tasim's inner circle, the prince of Ushrusana, al-Afshin, fell foul of his enemies at court and was overthrown and killed in 840/1. The rise of the Turks would eventually result in the troubles of the "Anarchy at Samarra" and lead to the collapse of Abbasid power in the mid-10th century, but the ghilmān-based system inaugurated by al-Mu'tasim would be widely adopted throughout the Muslim world.

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