Abu Ya'qub Ishaq ibn Ahmad al-Sijistani () or al-Sijzi () was a 10th-century Persian Ismaili missionary active in the northern and eastern Iranian lands. His life is obscure, but he was a prolific writer, who played a crucial role in the infusion of Neoplatonic ideas into Isma'ili theology. Life Al-Sijistani's life is obscure, as references to him are found mostly in isolation in hostile Sunni heresiological works, while Isma'ili sources usually do not provide any details about him. He was given the nickname 'cottonseed' (Arabic: , Persian: ) in several near-contemporary non-Isma'ili works that mention him, but the origin and significance of it are unknown. What can be gleaned from the sources is that he was a senior missionary () in the Iranian lands of the eastern Islamic world. Nizam al-Mulk reports that a certain Ishaq succeeded Abu Hatim al-Razi as chief at Rayy upon the latter's death on 934, while Ibn al-Nadim mentions a certain Abu Ya'qub ase chief at Rayy in . This Abu Ya'qub was also in charge of the missionary movement () in Upper Mesopotamia and Iraq, with the brothers Abu Muslim and Abu Bakr ibn hammad in Mosul and Ibn Nafis in Baghdad as his agents. This is likely to have been the same person as al-Sijistani, as it agrees with a statement in one of al-Sijistani's works that he was in Iraq in 934. Earlier opinion among scholars was that he was executed along with Muhammad al-Nasafi in 943, but this is now disproven. In reality, he was al-Nasafi's successor, both as chief in Khurasan, as well as in continuing the development of al-Nasafi's theological ideas. Other sources maintain that he was active in Sijistan (whence his ), both during and after al-Nasafi's tenure. The movement he headed was not initially affiliated with the Fatimid Caliphate, but at some point, during the caliphate of al-Mu'izz li-Din Allah (), he accepted the Fatimids as the legitimate imams, and many of his views were taken over by the Fatimid-sponsored . According to Rashid al-Din Hamadani, al-Sijistani was executed by the Saffarid emir of Sijistan, Khalaf ibn Ahmad (). Al-Sijistani's work Kitāb al-iftikhār was written around 971, this provides a terminus post quem for his execution. The introductions to two of his other works indicate they were written during the reign of the Fatimid caliph al-Hakim bi-Amr Allah (), but they are likely later interpolations. Al-Sijistani died