Islam is the majority religion in Azerbaijan, but the country is considered to be the most secular in the Muslim world. Various reports have estimated 97.3% or 99.2% of the population identifying as Muslim; with the majority being adherents of the Shia branch (55-65%), while a significant minority (35-45%) are Sunnis. Traditionally, the differences between these two branches of Islam have not been sharply defined in Azerbaijan.

Shia 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 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 Muhammad's other companions at Saqifah, to be the first rightful (rāshidūn) caliph after Muhammad. Adherents of Shīʿa Islam are called Shia Muslims.