Theodosius III or Theodosios III (Greek: Θεοδόσιος) was Byzantine emperor from c. May 715 to 25 March 717. Before rising to power and seizing the throne of the Byzantine Empire, he was a tax collector in Adramyttium. In 715, the Byzantine navy and the troops of the Opsician Theme (Byzantine province) revolted against Byzantine Emperor Anastasios II (r. 713–715), acclaiming the reluctant Theodosius as Emperor Theodosius III. Theodosius led his troops to Chrysopolis and then Constantinople, the capital, seizing the city in November 715. Anastasios did not surrender until several months later, accepting exile into the monastery in return for safety. Many themes refused to recognize the legitimacy of Theodosius, believing him to be a puppet of the troops of the Opsician Theme, especially the Anatolics and the Armeniacs under their respective strategoi (generals) Leo the Isaurian and Artabasdos.

Leo declared himself emperor in the summer of 716 and allied himself with the Umayyad Caliphate, an Islamic empire; Theodosius allied himself with the Bulgarians under Khan Tervel (r. 700–721), setting a firm border at Thrace, ceding the Zagoria region to the Bulgarians, as well as stipulating tribute payment to the Bulgarians. Leo then marched his troops to Constantinople, seizing the city of Nicomedia, capturing many officials, including Theodosius's son. With his son in captivity, Theodosius took the advice of Patriarch Germanus and the Byzantine Senate, and negotiated with Leo in spring 717, agreeing to abdicate and recognize Leo as emperor. Leo entered Constantinople and definitively seized power on 25 March 717, allowing Theodosius and his son to retire to a monastery as monks. Theodosius became bishop of Ephesus, and died at some point after, possibly on 24 July 754.