Sa'adiah ben Yosef Gaon (רבי סעדיה בן יוסף אלפיומי גאון; Arabic: سعيد بن يوسف الفيومي‎ / Saʻīd bin Yūsuf al-Fayyūmi, Sa'id ibn Yusuf al-Dilasi, Saadia ben Yosef aluf, Sa'id ben Yusuf ra's al-Kull‎; alternative English Names: Rabbeinu Sa'adiah Gaon ("our Rabbi [the] Saadia Gaon"), often abbreviated RSG (RaSaG), Saadia b. Joseph, Saadia ben Joseph or Saadia ben Joseph of Faym or Saadia ben Joseph Al-Fayyumi; (882/892 – 942) was a prominent rabbi, Gaon, Jewish philosopher, and exegete of the Geonic period who was active in the Abbasid Caliphate.

The first important rabbinic figure to write extensively in Arabic, he is considered the founder of Judeo-Arabic literature. Known for his works on Hebrew linguistics, Halakhah, and Jewish philosophy, he was one of the more sophisticated practitioners of the philosophical school known as the "Jewish Kalam" (Stroumsa 2003). In this capacity, his philosophical work The Book of Beliefs and Opinions represents the first systematic attempt to integrate Jewish theology with components of Greek philosophy. Saadia was also very active in opposition to Karaism, in defense of rabbinic Judaism.