Theodorus of Cyzicus was a late antique author of the fifth to sixth century CE. He is known only by his ethnonym, which identifies his origin as Cyzicus in the province of Hellespontus. No biographical details of his life or education survive.

His sole extant work is the Epistle to the Alexandrians, preserved in the acts of the Second Council of Nicaea in 787 CE. This Christological letter defends the doctrine that Christ assumed a complete human nature, including a rational soul, arguing against the fourth-century teachings of Apollinarius of Laodicea.

Theodorus’s significance stems from the later use of his letter at the Second Council of Nicaea. The council fathers cited it as a patristic authority against the Iconoclasts, interpreting its defense of Christ’s full humanity as theological justification for the depiction of Christ in icons. Thus, his anti-Apollinarian text was repurposed for the eighth-century iconoclastic controversy.