Saʿadiah ben Yosef Gaon (سعيد بن يوسف الفيومي Saʿīd bin Yūsuf al-Fayyūmi; סַעֲדְיָה בֶּן יוֹסֵף אַלְפַיּוּמִי גָּאוֹן Saʿăḏyāh ben Yōsēf al-Fayyūmī Gāʾōn; alternative English names: Rabbeinu Saʿadiah Gaon ("our Rabbi [the] Saadia Gaon"), often abbreviated RSG (RaSaG); Saadia b. Joseph; Saadia ben Joseph; Saadia ben Joseph of Faym; or Saadia ben Joseph Al-Fayyumi; 882/892 – 942) was a prominent rabbi, gaon, Jewish philosopher, and exegete who was active in the Abbasid Caliphate.
Saadia is the first important rabbinic figure to write extensively in Judeo-Arabic. Known for his works on Hebrew linguistics, Halakha, and Jewish philosophy, he was a practitioner of the philosophical school known as the "Jewish Kalam". In this capacity, his philosophical work The Book of Beliefs and Opinions represents the first systematic attempt to integrate Jewish theology with components of ancient Greek philosophy. Saadia was also very active in opposition to Karaite Judaism in defense of rabbinic Judaism.
Saadia was born in Dilâẓ, in the district of Faiyum, Middle Egypt, in 892 CE. He immigrated to Palestine in 915, at the age of 23, where he studied in Tiberias under the scholar Abu Kathir Yaḥya al-Katib, a Jewish theologian (mutakallim) mentioned also by ibn Ḥazm. In 926, Saadia settled permanently in Abbasid Iraq, known to Jews as "Babylonia", where he became a member of Sura Academy.
Saadia, in Sefer ha-Galui, stresses his Jewish lineage, claiming to belong to the noble family of Shelah, son of Judah, and counting among his ancestors Hanina ben Dosa, the famous ascetic of the first century. Expression was given to this claim by Saadia in calling his son Dosa (this son, Dosa ben Saadia, later served as Gaon of Sura from 1012–1018). Regarding Joseph, Saadia's father, a statement of Aaron ben Meir has been preserved saying that he was compelled to leave Egypt and died in Jaffa, probably during Saadia's prolonged residence in the Holy Land.