Abū al-ʿAbbās Abd Allāh ibn Hārūn al-Maʾmūn (Arabic: أبو العباس عبد الله بن هارون الرشيد, romanized: Abū al-ʿAbbās ʿAbd Allāh ibn Hārūn ar-Rashīd; 14 September 786 – 9 August 833), better known by his regnal name al-Ma'mun (Arabic: المأمون, lit. 'the Reliable'), was the seventh Abbasid caliph, who reigned from 813 until his death in 833. His leadership was marked by the power and prosperity of the Abbasid Caliphate, al-Ma'mun promoted the Graeco-Arabic translation movement, the flowering of learning and the sciences in Baghdad, and the publishing of al-Khwarizmi's book now known as "Algebra", making him one of the most important caliphs in the Islamic Golden Age. He is also known as a proponent of the rational Islamic theology of Mu'tazilism.

Al-Ma'mun succeeded his half-brother al-Amin after a civil war, much of his reign was spent on peace campaigns. His strong support for Mu'tazilism led him to imprison a Sunni Imam, Ahmad ibn Hanbal in an event that became known as mihna. Al-Ma'mun's foreign policy was due to his decision to continue war and diplomatic relations with the Byzantine Empire, the tension between conflict or diplomacy varying during his military campaigns.

Abdallah, the future al-Ma'mun, was born in Baghdad on the night of 13 to 14 September 786 CE to Harun al-Rashid and his concubine Marajil, from Badghis. On the same night, which later became known as the "night of the three caliphs", his uncle al-Hadi died and was succeeded by Ma'mun's father, Harun al-Rashid, as ruler of the Abbasid Caliphate. Marajil died soon after his birth, and Abdallah was raised by Harun al-Rashid's wife, Zubayda, herself of high Abbasid lineage as the granddaughter of Caliph al-Mansur (r. 754–775). As a young prince, Abdallah received a thorough education: al-Kisa'i tutored him in classical Arabic, Abu Muhammad al-Yazidi in adab, and he received instruction in music and poetry. He was trained in fiqh by al-Hasan al-Lu'lu'i, showing particular excellence in the Hanafi school, and in the hadith, becoming himself active as a transmitter. According to M. Rekaya, "he was distinguished by his love of knowledge, making him the most intellectual caliph of the Abbasid family, which accounts for the way in which his caliphate developed".

Although Abdallah was the oldest of his sons, in 794 Harun named the second-born Muhammad, born in April 787 to Zubayda, as the first in line of succession. This was the result of family pressure on the Caliph, reflecting Muhammad's higher birth, as both parents descended from the Abbasid dynasty; indeed, he remained the only Abbasid caliph to claim such descent. Muhammad received the oath of allegiance (bay'ah) with the name of al-Amin ("The Trustworthy"), first in Khurasan by his guardian, the Barmakid al-Fadl ibn Yahya, and then in Baghdad. Abdallah was recognized as second heir only after entering puberty, in 799, under the name al-Ma'mun ("The Trusted One"), with another Barmakid, Ja'far ibn Yahya, as his guardian. At the same time, a third heir, al-Qasim, named al-Mu'tamin, was appointed, under the guardianship of Abd al-Malik ibn Salih.

These arrangements were confirmed and publicly proclaimed in 802, when Harun and the most powerful officials of the Abbasid government made the pilgrimage to Mecca. Al-Amin would succeed Harun in Baghdad, but al-Ma'mun would remain al-Amin's heir and would additionally rule over an enlarged Khurasan. This was an appointment of particular significance, as Khurasan had been the starting point of the Abbasid Revolution which brought the Abbasids to power, and retained a privileged position among the Caliphate's provinces. Furthermore, the Abbasid dynasty relied heavily on Khurasanis as military leaders and administrators. Many of the original Khurasani Arab army (Khurasaniyya) that came west with the Abbasids were given estates in Iraq and the new Abbasid capital, Baghdad, and became an elite group known as the abnāʾ al-dawla ("sons of the state/dynasty"). This large-scale presence of an Iranian element in the highest circles of the Abbasid state, with the Barmakid family as its most notable representatives, was certainly a factor in the appointment of al-Ma'mun, linked through his mother with the eastern Iranian provinces, as heir and governor of Khurasan. The stipulations of the agreement, which were recorded in detail by the historian al-Tabari, accorded al-Mamun's Khurasani viceroyalty extensive autonomy. However, modern historians consider that these accounts may have been distorted by later apologists of al-Ma'mun in the latter's favour. Harun's third heir, al-Mu'tamin, received responsibility over the frontier areas with the Byzantine Empire in Upper Mesopotamia and Syria.

Very quickly, the latent rivalry between the two brothers had important repercussions: almost immediately after the court returned to Baghdad in January 803, the Abbasid elites were shaken by the abrupt fall of the Barmakid family from power. On the one hand, this event may reflect the fact that the Barmakids had become indeed too powerful for the Caliph's liking, but its timing suggests that it was tied to the succession issue as well: with al-Amin siding with the abnāʾ and al-Ma'mun with the Barmakids, and the two camps becoming more estranged every day, if al-Amin was to have a chance to succeed, the power of the Barmakids had to be broken.

Al-Fadl ibn Sahl, a Kufan of Iranian origin whose father had converted to Islam and entered Barmakid service, replaced Ja'far ibn Yahya as al-Ma'mun's tutor. In 806 he also became al-Ma'mun's secretary (katib), an appointment that marked him out as the chief candidate for the vizierate should al-Ma'mun succeed to the throne. In 804, al-Ma'mun married his cousin, Umm Isa, a daughter of the Caliph al-Hadi (r. 785–786). The couple had two sons, Muhammad al-Asghar and Abdallah.

The years after the fall of the Barmakids saw an increasing centralization of the administration and the concomitant rise of the influence of the abnāʾ, many of whom were now dispatched to take up positions as provincial governors and bring these provinces under closer control from Baghdad. This led to unrest in the provinces, especially Khurasan, where local elites had a long-standing rivalry with the aabnāʾ and their tendency to control the province (and its revenues) from Iraq. The harsh taxation imposed by a prominent member of the abnāʾ, Ali ibn Isa ibn Mahan, even led to a revolt under Rafi ibn al-Layth, which eventually forced Harun himself, accompanied by al-Ma'mun and the powerful chamberlain (hajib) and chief minister al-Fadl ibn al-Rabi, to travel to the province in 808. Al-Ma'mun was sent ahead with part of the army to Merv, while Harun stayed at Tus, where he died on 24 March 809.