In Christian theology, a heresiarch (also hæresiarch, according to the Oxford English Dictionary; from Greek: αἱρεσιάρχης, hairesiárkhēs via the late Latin haeresiarcha) or arch-heretic is an originator of heretical doctrine or the founder of a sect that sustains such a doctrine.

In his Divine Comedy, Dante Alighieri represents the heresiarchs as being immured in tombs of fire in the Sixth Circle of Hell. In Cantos IX and X of the Inferno, Virgil describes the suffering these souls experience, saying "Here are the Arch-Heretics, surrounded by every sect their followers... / Like with like is buried, and the monuments are different in degrees of heat." Among the historical figures that Dante specifically lists as arch-heretics are Epicurus, Farinata Degli Uberti, Frederick II of Sicily, and Pope Anastasius II.