Abdallah Ibn Yasin (died 7 July 1059 C.E. in "Krifla" near Rommani, Morocco) was a theologian and founder of the Almoravid movement and dynasty.
Abdallah ibn Yasin was from the tribe of the Jazulah (pronounced Guezula), a Sanhaja sub-tribe from the Sous. He was a Maliki theologian, he was a disciple of Waggag ibn Zallu al-Lamti and studied in his Ribat, "Dar al-Murabitin" which was located in the village of Aglu, (near present-day Tiznit). In 1046 the Gudala chief Yahya Ibn Ibrahim, came to the Ribat asking for someone to promulgate Islmaic religious teachings amongst the Berber of the Adrar (present-day Mauritania) and Waggag ibn Zallu chose to send Abdallah ibn Yasin with him. The Sanhaja were at this stage only superficially Islamicised and still clung to many heathen practices, and so Ibn Yasin preached to them an orthodox Sunnism.
After a revolt of the Gudala he was forced to withdraw with his followers. In alliance with Yahya ibn Umar, the leader of the Lamtuna tribe, he managed to quell the rebellion.
Ibn Yasin now formed the Almoravid alliance from the tribes of the Lamtuna, the Masufa and the Gudala, with himself as spiritual leader and Yahya ibn Umar taking the military command. In 1054 the Maghrawa-ruled Sijilmasa was conquered. Ibn Yasin introduced his orthodox rule - amongst other things wine and music were forbidden, non-Islamic taxes were abolished and one fifth of the spoils of war were allocated to the religious experts. This rigorous application of Islam soon provoked a revolt in 1055.
Yahya ibn Umar was killed in 1056 in a renewed revolt of the Gudala in the Sahara, upon which Ibn Yasin appointed Yahya's brother Abu-Bakr Ibn-Umar (1056–1087) the new military leader. Abu Bakr destroyed Sijilmasa, but was not able to force the Gudala back into the Almoravid league. He went on to capture Sūs and its capital Aghmat (close to where is now Marrakech) in 1057.
Ibn Yasin died while attempting to subjugate the Barghawata on the Atlantic coast in 1059. His grave is 33 km almost due south of Rabat, near Rommani, overlooking the Krifla River, marked on Michelin maps as the marabout of Sidi Abdallah. A mosque and a mausoleum were built on his grave, the site is still intact today.