Ismāʿīlism (Arabic: الإسماعيليون‎ al-Ismāʿīliyyūn; Persian: اسماعیلیان‎Esmāʿiliyān; Urdu: إسماعیلی Ismāʿīlī) is the second largest branch of Shia Islam after the Twelvers (Ithnāʿashariyya). The Ismāʿīlī get their name from their acceptance of Ismāʿīl ibn Jaʿfar as the appointed spiritual successor (Imām) to Jaʿfar aṣ-Ṣādiq, wherein they differ from the Twelvers, who accept Mūsà al-Kāżim, younger brother of Ismāʿīl, as the true Imām.

Tracing its earliest theology to the lifetime of Muḥammad, Ismāʿīlism rose at one point to become the largest branch of Shī‘ism, climaxing as a political power with the Fatimid Empire in the tenth through twelfth centuries. Ismailis believe in the oneness of God, as well as the closing of divine revelation with Muḥammad, whom they see as the final Prophet and Messenger of God to all humanity. The Ismāʿīlī and the Twelvers both accept the same initial A'immah from the descendants of Muḥammad through his daughter Fāṭimat-al-Zahra and therefore share much of their early history. Both Shī‘ite groups see the family of Muḥammad (Ahl al-Bayt) as divinely chosen, infallible (ismah), and guided by God to lead the Islamic community (Ummah), a belief that distinguishes them from the majority Sunni branch of Islam.