Isma'ilism (Arabic: الإسماعيلية, romanized: al-ʾIsmāʿīlīyah) is a branch or sub-sect of Shia Islam. The Isma'ili () get their name from their acceptance of Imam Isma'il ibn Jafar as the appointed spiritual successor (imām) to Ja'far al-Sadiq, wherein they differ from the Twelver Shia, who accept Musa al-Kadhim, the younger brother of Isma'il, as the true Imām.

Isma'ilism rose at one point to become the largest branch of Shia Islam, climaxing as a political power with the Fatimid Caliphate in the 10th through 12th centuries. Ismailis believe in the oneness of God, as well as the closing of divine revelation with Muhammad, whom they see as "the final Prophet and Messenger of God to all humanity". The Isma'ili and the Twelvers both accept the same six initial Imams; the Isma'ili accept Isma'il ibn Jafar as the seventh Imam.

After the death of Muhammad ibn Isma'il in the 8th century CE, the teachings of Ismailism further transformed into the belief system as it is known today, with an explicit concentration on the deeper, esoteric meaning (batin) of the Islamic religion. With the eventual development of Usulism and Akhbarism into the more literalistic (zahir) oriented, Shia Islam developed into two separate directions: the metaphorical Ismaili, Alevi, Bektashi, Alian, and Alawite groups focusing on the mystical path and nature of God, along with the "Imam of the Time" representing the manifestation of esoteric truth and intelligible divine reality, with the more literalistic Usuli and Akhbari groups focusing on divine law (sharia) and the deeds and sayings (sunnah) of Muhammad and the Twelve Imams who were guides and a light to God.

Isma'ili thought is heavily influenced by Neoplatonism.

The larger sect of Ismaili are the Nizaris, who recognize Aga Khan IV as the 49th hereditary Imam, while other groups are known as the Tayyibi branch. The biggest Ismaili community is in Gorno-Badakhshan, but Isma'ilis can be found in Central Asia, Afghanistan, Pakistan, Yemen, Lebanon, Malaysia, Syria, Iran, Saudi Arabia, India, Jordan, Iraq, Kuwait, East Africa, Angola, Bangladesh, and South Africa, and have in recent years emigrated to Europe, Canada, Australia, New Zealand, the United States, and Trinidad and Tobago.