Twelver (Arabic: اثنا عشرية‎, Athnā‘ashariyyah or Ithnā‘ashariyyah; Persian: شیعه دوازده‌ امامی‎‎, pronounced [ʃiːʔe-je dævɑzdæh emɑmiː]) Shia Islam or Imamiyyah (Arabic: إمامية‎) is the largest branch of Shia Islam. The term Twelver refers to its adherents' belief in twelve divinely ordained leaders, known as the Twelve Imams, and their belief that the last Imam, Muhammad al-Mahdi, lives in occultation and will reappear as the promised Mahdi.

Twelvers believe that the Twelve Imams are the spiritual and political successors to the Islamic prophet Muhammad. According to the theology of Twelvers, the Twelve Imams are exemplary human individuals who not only rule over the community in justice, but also are able to keep and interpret sharia and the esoteric meaning of the Quran. The words and deeds (Sunnah) of Muhammad and the Imams are a guide and model for the community to follow; as a result, Muhammad and the Imams must be free from error and sin, known as Ismah or infallibility, and must be chosen by divine decree, or nass, through Muhammad.