Theophylact or Theophylaktos (Greek: Θεοφύλακτος; c. 793 – 15 January 849) was the eldest son of the Byzantine emperor Michael I Rhangabe (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.