Shīʿa Islam (), otherwise known as Shīʿism () or as Shīʿite or Shīʿī Islam (, ), is the second-largest branch of Islam. It holds that the Islamic prophet Muhammad designated ʿAlī ibn Abī Ṭālib as his successor (khalīfa) and the Imam (spiritual and political leader) after him, most notably at the event of Ghadir Khumm, but was prevented from succeeding Muhammad as the leader of the Muslims as a result of the choice made by some of Muhammad's other companions (ṣaḥāba) at Saqifah. This view primarily contrasts with that of Sunnī Islam, whose adherents believe that Muhammad did not appoint a successor before his death and consider Abū Bakr, who was appointed caliph by a group of senior Muslims at Saqifah, to be the first rightful (rāshidūn) caliph after Muhammad. Adherents of Shīʿa Islam are called Shīʿa Muslims, Shīʿites, or simply Shīʿa, Shia, or Shīʿīs.

Shīʿa Islam is based on a ḥadīth report concerning Muhammad's pronouncement at Ghadir Khumm. Shīʿa Muslims believe that ʿAlī ibn Abī Ṭālib, Muhammad's cousin and son-in-law, should have been the designated successor to Muhammad as Islam's spiritual and political leader. This belief later developed into the concept of Imamah, the idea that certain descendants of Muhammad, the Ahl al-Bayt, are rightful rulers or Imams, whom Shīʿa Muslims believe possess special spiritual and political authority over the Muslim community. Although there are many Shīʿa subsects, modern Shīʿa Islam has been divided into two main groupings: Twelvers and Ismāʿīlīs, with Twelver Shīʿas being the largest and most influential group among Shīʿa Muslims.

Shīʿa Islam is the second largest branch of Islam, followed by 10–15% of all Muslims. Twelver Shīʿīsm is the largest branch of Shīʿa Islam, comprising about 85% of all Shīʿa Muslims.