a grandson of the Prophet Muhammad (the founder of Islam), the elder son of Muhammad's daughter Fāṭimah. He belongs to the group of the five most holy persons of Shīʿah, those over whom Muhammad spread his cloak while calling them “The People of the House.” After his father, Alīʿ, he was considered by many of his contemporaries to be the rightful heir to Muhammad's position of leadership.

As a child, Ḥasan lived with Muhammad for seven years, and after the latter's death in 632 he was politically inactive until the end of the reign of the caliph Uthmān ibn ʿAffānʿ (the caliph was the titular leader of the Islamic community). ʿUthmān was murdered in 656, an action in which Ḥasan took no part. ʿAlī, Ḥasan's father, became the next caliph, and in the civil wars that soon broke out Ḥasan was sent to the important Iraqi city of Kūfah to secure acceptance of ʿAlī's rule and, if possible, obtain military reinforcements. Later he fought in the Battle of Ṣiffīn (Ṣiffīn, Battle of), which, although not a defeat, did mark the beginning of a steady deterioration in ʿAlī's position. After ʿAlī was murdered in 661, never having chosen a successor, a large number of his followers pledged their loyalty to Ḥasan, and Ḥasan himself stressed his own close connections with the Prophet Muhammad.

When Muʿāwiyah I, the governor of Syria and the man who had led the rebellion against ʿAlī, refused to acknowledge Ḥasan as caliph and began to prepare for war, Ḥasan was able to offer considerable resistance: he dispatched a force to meet Muʿāwiyah and then himself headed a larger force. With little money left, Ḥasan, not a warlike person, was plagued by defections from his army. Although some of his followers resented it fiercely, he opened peace negotiations and later in 661 abdicated the caliphate to Muʿāwiyah. Ḥasan ibn ʿAlī obtained a generous pension and was allowed to live quietly the rest of his life in Medina.