Theli (Hebrew: תְּלִי‎, Təlī; also translated as Tali, Thele, T'li, etc.), according to the Sefer Yetzirah, the earliest extant work of Jewish mysticism, is a celestial being who surrounds the universe. Theli is briefly mentioned in two verses^[1] of the Sefer Yetzirah, where he is described as "above the universe, as a king on his throne".^[1] However, his overall purpose is never elaborated upon, and the Sefer Yetzirah itself does not attribute a particular form to Theli. Despite this, in the few sources which mention him, Theli is generally envisioned as a dragon.

Shabbethai Donnolo, in his commentary on the Sefer Yetzirah, the Hakhmoni, envisioned Theli as a "cosmic dragon" which possessed great power within the universe, and governed both planets and constellations.^[2] Saadia Gaon, in his own commentary on the Sefer Yetzirah, sought to explain Theli in purely astronomical terms, and recognized he was not merely a constellation resembling a dragon,^[3] although Isidor Kalisch identified Theli with Draco.^[4] Ibn Ezra remarked that he was uncertain of what Theli constituted, instead identifying him with Leviathan as a tannin, or sea monster.^[5] The Vilna Gaon shares this identification of Theli as a tannin, while still asserting its supremacy over the signs of the zodiac.^[6]

The etymology of Theli is uncertain, and the term does not resemble tannin, nakhash (“serpent”), or any other Hebrew word which became associated with “dragon” in later Jewish thought.