Berber caliph of the Almohad (Almohads) dynasty (reigned 1130–63), who conquered the North African Maghrib from the Almoravids andbrought all the Berbers under one rule.

ʿAbd al-Muʾmin came from a humble family: his father had been a potter. He seems to have been well instructed in the Muslim faith and must have had a good knowledge of Arabic, for he wished to continue his studies at one of the centres of Muslim learning in the East. A chance meeting with Ibn Tūmart, a Berber religious reformer, made him abandon this idea and begin his brilliant career.

Around 1117, Ibn Tūmart, the founder of the Almohad movement, was returning from a long stay in the East. He landed at Mahdīyah in Tunisia and began a journey to southern Morocco, his native country. Wherever he stopped along the way, he proclaimed a twofold message: strict adherence to the doctrine of the oneness of God (hence the name Almohads or al-Muwaḥḥidūn, Unitarians) and scrupulous observance of Islāmic law. ʿAbd al-Muʾmin heard Ibn Tūmart preach at Mellala, near Bejaïa, Algeria. He was an attentive listener and from that time attached himself to the man who had revealed to him the true doctrine.

ʿAbd al-Muʾmin does not seem to have played any special role among Ibn Tūmart's disciples during the slow journey that took them to Marrakech. But when his master declared his opposition to the ruling Almoravid (Almoravids) regime, proclaimed himself the mahdī (“divinely guided one”), and took refuge in the remote High Atlas region, ʿAbd al-Muʾmin went with him. Ibn Tūmart won a following in the mountains and founded a small Almohad state there, centred on the village of Tinmel. When al-Bashīr, the reformer's second in command, was killed in an attack on Marrakech, ʿAbd al-Muʾmin took his place and became Ibn Tūmart's designated successor. The mahdī died in 1130. His death was kept secret at first to allow ʿAbd al-Muʾmin—a stranger to the High Atlas—time to win support from the Almohad leaders. When he was proclaimed leader of the Almohads, he assumed the prestigious title of caliph.

His first task was to carry on the struggle against the Almoravids. Learning from the failure at Marrakech, he realized that he must conquer Morocco from the mountains. On the plains, the Christian knights who served the Almoravids could easily repulse the Almohads' Berber infantry. He spent the next 15 years winning control of the High Atlas, Middle Atlas, and Rif regions, finally moving into his native country, north of Tlemcen.

Near that town, the Almoravids, having suffered the loss of Reverter, the leader of their Catalan mercenaries, were defeated by ʿAbd al-Muʾmin in open battle in 1145. The Almohad forces then moved west, subjugating Morocco's Atlantic coastal plain. They then laid siege to Marrakech and took it by storm in 1147, massacring the Almoravid inhabitants.

Arab historians have left a description of the man who had now become master of Northwest Africa. He was a sturdy Berber of medium height, with dark hair and regular features. A good soldier, with great courage and endurance, he was at the same time learned in Islām and a gifted orator. Although he had personal charm and could, when necessary, show patience and moderation, he was at times as harsh as his master, Ibn Tūmart. When a revolt broke out in the Atlantic plain area following the capture of Marrakech, he conducted a methodical purge there in which more than 30,000 people were executed.

ʿAbd al-Muʾmin left neither memoirs nor a political testament; his ideas must be deduced from his actions. His newfound power and his very success raised problems that demanded immediate solutions.

The capture of Marrakech posed the moral question of whether to abandon this city founded by the Almoravid heretics, whom he had exterminated without pity. He contented himself with destruction of their palace and mosques and retained Marrakech as the capital of his new empire.

Soon he had to choose between two imperial policies: to complete the conquest of North Africa or to concentrate his energies on Spain, where the Christians were threatening the former Almoravid domains. Showing good judgment as well as feeling for his native country, he gave priority to North Africa.

In 1151 he subjugated the area around Constantine and on his way home fought a battle near Sétif against a powerful coalition of Arab tribes that had been wandering over the Berber country for a century, gradually destroying its simple, pastoral, and sedentary way of life. ʿAbd al-Muʾmin was victorious, but instead of punishing these people who had showed themselves to be the worst enemies of the Berbers and the Almohad government, he came to rely on them to strengthen his dynasty against internal opposition from the family of Ibn Tūmart. He also wished to use the Arab cavalry in his holy war against the Christians in Spain.

In 1158–59 ʿAbd al-Muʾmin conquered Tunisia and Tripolitania. This marked the zenith of Berber power in Islām: a Berber caliph reigned over all of North Africa west of Egypt, and his authority was acknowledged by most of Muslim Spain as well.

Even while he was pursuing his conquest, ʿAbd al-Muʾmin had established a central government for his empire. To the traditional clan organization of the Maṣmudah and other Berber peoples supporting the Almohads he added an organization to promote the spread of Almohad doctrine and a central administration (the makhzan) modeled on those of Muslim Spain, which was staffed largely by Spanish Muslims. A government land registry was improvised to assure the dynasty regular revenue. ʿAbd al-Muʾmin fully accepted the responsibilities of an art patron, but remembering the puritanical austerity of Ibn Tūmart, he sometimes imposed on the mosques built for him by Andalusian artisans a plainness that became more precious than the prevailing elaborate ornamentation.

ʿAbd al-Muʾmin died in 1163. His work, faithfully carried on by his successors Abū Yaʿqūb Yūsuf (reigned 1163–84) and Abū Yūsuf Yaʿqūb al-Manṣūr (1184–99), was maintained for more than half a century. Disturbances caused by the rebellious Arab tribes impoverished the country without endangering the dynasty. After their defeat by the Spanish Christians at Las Navas de Tolosa in 1212, however, the Almohads began to decline, and their empire soon disintegrated.

Though in the long run ʿAbd al-Muʾmin's successors proved unable to perpetuate his achievements, he himself had written one of the most glorious chapters in the history of the Muslim West.