Ja'far's death in 765 produced one of the decisive divisions of Shia history. His designated successor, his son Isma'il, appears to have died before him, and the majority followed another son, Musa al-Kazim, as the seventh Imam, the line that Twelver Shia Islam continues to the twelfth. But a body of followers held that the Imamate had passed through Isma'il and his descendants, and from them descend the Ismailis, the branch that would later found the Fatimid caliphate and that survives in several communities today. The split turns directly on the succession to Ja'far.
A quite different afterlife attached his name to the esoteric sciences. Later alchemical and occult literature claimed him as a master: the vast corpus of works ascribed to the alchemist Jabir ibn Hayyan presents Jabir as Ja'far's pupil, and traditions of lettrism, divination and the properties of things were fathered upon him. Modern scholarship regards these attributions as largely legendary, the borrowing of an authoritative name by later writers, much as happened with Solomon and Hermes.
Across law, theology and legend alike, Ja'far al-Sadiq stands as one of the pivotal transmitters of early Islamic learning, honoured by Sunni jurists and revered by Shia as their Imam.