Theodosius III, (Greek Θεοδόσιος Γ') emperor of the Byzantine Empire (715-717), was a financial officer and tax collector in Adramyttium before being acclaimed in May of 715 as an imperial candidate for the troops of the Opsikian theme rebelling against Anastasius II.
According to the chronicler Theophanes, Theodosius was unwilling to accept the honor, but found himself unable to reject it. The future emperor and his troops immediately laid siege to Constantinople. Six months later, in November, they captured the city and deposed Anastasius.
Little is known of Theodosius' short reign. He immediately faced rebellions in Anatolia and invasion by an Arab fleet. In 716 he concluded a treaty with the Khan Tervel favorable to the Bulgarians in effort to repel the Arab invasion.
In 717, Leo the Isaurian (the future Leo III) rebelled against Theodosius' rule. Theodosius' son was captured by Leo in Nicomedia, and Theodosius chose to resign the throne on March 25, 717. He and his son subsequently entered the clergy.
By 729 Theodosius is believed to have become bishop of Ephesus. Modern historians however suspect the bishop was actually Theodosius' namesake son. Either way, this bishop was last recorded alive on July 24, 754, taking part in the iconoclastic Council of Hieria.