Abu Ishaq al-Mu'tasim ibn Harun (794 - January 5, 842) was an Abbasid caliph who reigned from 833 until 842). He succeeded his half-brother al-Ma'mun.

The ghilman (sing., ghulam) were introduced to the Caliphate during al-Mu'tasim's reign. The ghilman were slave-soldiers taken as children from conquered regions, in anticipation of the Ottoman devshirme system, and made into soldiers. The ghilman, personally responsible only to the Caliph, were to revolt during the reign of al-Radi.

The ghilman, along with the shakiriya which had been introduced in the reign of al-Ma'mun, had irritated the Arab regular soldiers of the Caliph's army. The Turkic and Armenian ghilman agitated the citizens of Baghdad, provoking riots in 836. The capital was moved to the new city of Samarra later that year, where it would remain until 892 when it was returned to Baghdad by al-Mu'tamid.

The Tahirid dynasty, which had come to prominence during al-Ma'mun's reign after the military province of Khurasan was granted to Tahir bin Husain, continued to grow in power. They received the governorships of Samarqand, Farghana , and Herat. Unlike most provinces in the Abbasid Caliphate, which were closely governed by Baghdad and Samarra, the provinces under the control of the Tahirids were exempted from many tributes and oversight functions. The independence of the Tahirids contributed greatly to the decline of Abbasid supremacy in the east.

It was during al-Mu'tasim's reign that the fractures in the Abbasid empire began to become truly apparent. al-Mu'tasim had to cope with the Khurramiyyah in the area of Tabriz. Led by the heretic Babak, the Khurramiyyah were never fully suppressed, although they slowly vanished during the reigns of succeeding Caliphs.

The great Arab mathematician al-Kindi was employed by al-Mu'tasim, and tutored the Caliph's son. al-Kindi had served at the Bayt al-Hikma, or House of Wisdom. He continued his studies in Greek geometry and algebra under the caliph's patronage.

al-Mu'tasim died in 842 and was succeeded by his son, al-Wathiq.