Theodosius III (-c. 754), also spelt Theodosios III, was a financial officer and tax collecter in the Byzantine Empire, with some claiming him to be the son of former Emperor Tiberios III. When the Opsikion troops rebelled against Emperor Anastasius II, they declared for Theodosius, a choice he did not readily accept. According to one story he even tried to hide in the forests near Adramyttium before found and acclaimed emperor in 715.

Theodosius and his troops laid siege to Constantinople, and gained entry into the capital six months later. He was remarkably moderate towards his predecessor and his supporters, convincing Anastasius to abdicate and become a monk in Thessalonica. As emperor, he immediately had to deal with an Arab invasion into Anatolia, and negotiated a peace treaty with the Bulgarians.

In 717, the strategos of the Anatolic Theme, Leo the Isaurian, rebelled against Theodosius and claimed the throne for himself. Rather than fight, Theodosius chose to resign, entering the clergy. Records of him after his resignation are scant, and he is last recorded alive in 754, attending the iconoclastic Council of Hieria.