Abū Ya'qūb al-Sijistānī (Persian: ابو یعقوب سجستانی) or Sijzi (fl. 971 CE) was a PersianIsmaili missionary and Neo-Platonicphilosopher, who died sometime around or after 971 CE (358 AH). Abū Ya'qūb al-Sijistānī most likely originally comes from the Persian Central Asian lands who has been the appointed a chief da'i of Sistan (Sijistan in Arabic). There are evidences that he was also active in northern Iraq and Khurasan. Sijistani was the student of al-Nasafi, a prominent Central Asian Ismaili dai who has been reported to have converted the Samanid shah Nasr II. Sijistani, in contrast to his teacher, accepted the Imamate of the Fatimid Imam around after 950 despite initially preaching the coming of Muhammad Ibn Ismaili as Mahdi alongside the Qarmatis. Al-Sajistani has been accredited by bringing Neo-Platonism, which was already initiated by al-Nasafi, to Fatimid Ismailism under the Fatimid Imams Ma'add al-Mu'izz li-Dīn Allāh and Abū Manṣūr Nizār al-'Azīz bi-llāh.
His works were studied by the Ismailis until modern times. Among his works, only Ithbat al-Nubuwat, Yanabi', and Kashf al-Mahjub have been published; the last one has survived only in a Persian version, which has been translated to French by Henry Corbin.