a sect of the Shīʿites (one of the major branches of Islām) that was most active as a religio-political movement in the 9th–13th century through its subsects, the Fāṭimids, the Qarāmiṭah (Qarmatians), and the Assassins.

The Ismāʿīlītes came into being after the death of Jaʿfar ibn Muḥammad (765), the sixth imam, or spiritual successor to the Prophet, who was recognized by the Shīʿites. Jaʿfar's eldest son, Ismāʿīl, was accepted as his successor only by a minority, who became known as the Ismāʿīlītes. Those who accepted Jaʿfar's younger son, Mūsā al-Kāẓim, as the seventh imam and acknowledged his successors through the 12th imam became known as the Ithnā ʿAsharīyah, or Twelvers, the largest and most conservative of the Shīʿite sects. Certain of the Ismāʿīlītes (known as Wāqifīyah, or Stoppers) believed Ismāʿīl to have been the seventh and last imam and were designated as Seveners (Sabʿīyah), while the majority of Ismāʿīlītes believed the imamate continued in the line of the Fāṭimid caliphs. The Seveners later claimed that Ismāʿīl's son Muḥammad at-Tamm was expected to return at the end of the world as the mahdi (“divinely guided one”).

Ismāʿīlīte doctrine, formulated during the late 8th and early 9th centuries, stressed the dual nature of Qurʾanic interpretation, exoteric and esoteric, and, like Manichaeism, made a corresponding distinction between the ordinary Muslim and the initiated Ismāʿīlīte. The secret wisdom of the Ismāʿīlītes was accessible only through a hierarchical organization headed by the imam and was disseminated by dāʿīs (missionaries), who introduced believers into the elite through carefully graded levels. The Rasāʾil ikhwān aṣ-ṣafāʾ wa khillān al-wafāʾ (“Epistles of the Brethren of Purity and Loyal Friends”), a 10th-century philosophical and religious encyclopaedia influenced by Neoplatonism, was said to have been composed by a secret confraternity connected with the Ismāʿīlītes.

The Ismāʿīlītes became active in the second half of the 9th century in southern Iraq under the leadership of Ḥamdān Qarmaṭ. This branch of the sect, which came to be known as the Qarāmiṭah (Qarmatian), established itself in Iraq, Yemen, and especially Bahrain, in the 9th–11th century.

In Tunis, ʿUbayd Allāh established himself as the first Fāṭimid (Fāṭimid Dynasty) caliph in 909, claiming descent—through a line of “hidden imams”—from Muḥammad, son of Ismāʿīl, and through him from Fāṭimah, daughter of the Prophet, whence the dynastic name. The Fāṭimids conquered Egypt in 969; while they did not succeed in converting the bulk of their subjects during their brilliant rule of two centuries, they did create a widespread Ismāʿīlīte missionary network with followers all over the Islāmic world.

A fatal schism split the movement over the succession to the Fāṭimid caliph al-Mustanṣir (d. 1094). The Egyptian Ismāʿīlītes recognized his son al-Mustaʿlī, but the Ismāʿīlītes of Iran and Syria upheld the claims of his older son, Nizār; hence, there are two branches of Fāṭimids, the Mustaʿlīs and the Nizārīs.

When Ismāʿīlīyah came to an end in Egypt with the deposition of the last Fāṭimid caliph by Saladin in 1171, the Mustaʿlī Ismāʿīlītes survived in Yemen. They had not recognized any Fāṭimid after al-Āmir, al-Mustaʿlī's son, and believed that al-Āmir's infant son aṭṬayyib remained alive and that the line of the imams was hidden until a future time. In the interim they are governed by the chief dāʿī. In the 16th century the dāʿī of a major branch of the Mustaʿlis relocated in India and now resides in Surat, in Gujarāt district. His followers in India are usually known as Bohrās (Bohrā).

The Nizārīs, led by Ḥasan-e Ṣabbāḥ, gained control of a number of fortresses in Iran and Syria, the chief being Alamūt (1090). Known as Assassins, they remained in political power through the 13th century until displaced by the Mongols and the Mamlūks. The Nizārīs survived, though in two rival lines. The minor line died out by the 18th century, while the major line, led by an imam called the Aga Khan, moved from Iran to India in 1840. The Aga Khan has a following, estimated in the millions, in India and Pakistan and in parts of Iran, Africa, and Syria.

The Druze, a hill people living in modern southern Lebanon, neighbouring Syria, and Israel, separated from the main body of the Ismāʿīlītes early in the 11th century. They then formed a special closed religion of their own, which acknowledged the imams as incarnations of the godhead.