Theodosius III, emperor of the East (716-717), was a financial officer whom a Byzantine army rebelling against Anastasius II unexpectedly proclaimed monarch in his stead.

He captured Constantinople after a six months' siege and deposed Anastasius, but immediately faced rebellions in Anatolia and an invasion by an Arab fleet. The following year was himself forced to resign by a new usurper, Leo III. Theodosius ended his life in a monastery.