Natronai Ben Hilai HaKohen (Hebrew: נטרונאי בן הלאי or Natronai the Gaon, Hebrew: נטרונאי הגאון; Full name: Natronai ben Hilai ben Mari) was Gaon "head rabbi" of the Sura Academy of Rabbinic Judaism in Mesopotamia early in the second half of the 9th century. He held this post for ten years.^[1] He is responsible for more written responsa to queries posed to him by world Jewry than any of his predecessors, and maintained close contact with the Jews of al-Andalus (now Spain and Portugal).^[1]

Naṭronai was elderly when he became Gaon. Nonetheless, many responsa were issued by him. Questions were addressed to him from all parts of the Jewish diaspora, and his answers, about 300 of which have been preserved in various compilations (e.g., in Sha'are Ẓedeḳ, Teshubot ha-Ge'onim, and Ḳebuẓat ha-Ḥakamim, Tshuvot Rav Natrunai Gaon, Jerusalem 5771, edited by Y. Brody), show his thorough mastery of the subjects treated as well as his ability to impart knowledge.

Natronia always employed the language with which his correspondents were most conversant. He wrote equally well in Jewish Babylonian Aramaic and Medieval Hebrew and was the first attested Gaon to use Arabic (in the form of Jewish Middle Arabic for scholastic correspondence.

Natronai staunchly opposed Karaite Judaism. He endeavored to enforce the observance of every rabbinic provision emanating from or as explained by either of the Talmudic academies in Babylonia. Since the Karaites rejected the ritualistic forms of these schools, he made strenuous efforts to establish uniformity among the Rabbanites. The origin of many ritualistic formulae is traced to him.^[2]

Naṭronai was also credited with a mastery of kefitzat haderech "shortening of the route", or magical transit. He allegedly miraculously transported himself to France, where he instructed the people, and then returned to Mesopotamia in the same manner. Hai ben Sherira, however, denied this, suggesting that some adventurer may have impersonated Natronai and imposed on the Jews of France.^[3]