Theophylact or Theophylaktos (Greek: Θεοφύλακτος; c. 793 – 15 January 849) was the eldest son of the Byzantine emperor Michael I Rangabe (r. 811–813) and grandson, on his mother's side, of Nikephoros I (r. 802–811).

He was junior co-emperor alongside his father for the duration of the latter's reign, and was tonsured, castrated, and exiled to Plate Island after his overthrow, under the monastic name Eustratius.

[2] He was named after his paternal grandfather, the droungarios of the Dodekanesos Theophylact Rhangabe, who had participated in a failed conspiracy to wrest the throne from Empress-regent Irene of Athens in 780.

Despite a warm reception at Aachen and the ratification of a peace treaty between the two realms, Charlemagne, perhaps wary after the repeated failures of successive efforts to that effect over the previous decades, hesitated to agree to such a match.

[1][8] According to Theophanes Continuatus, Theophylact, who adopted the monastic name Eustratius (Εὐστράτιος), died five years after his father, on 15 January 849, and was buried alongside him in a church on Plate Island.