Anastasius I (Latin: Flavius Anastasius Dicorus Augustus, Greek: Ἀναστάσιος; c. 431 – 9 July 518) was Byzantine Emperor from 491 to 518. During his reign the Roman eastern frontier underwent extensive re-fortification, including the construction of Dara, a stronghold intended to counter the Persian fortress of Nisibis. During his reign, one of the most fortified castle cities on the Adriatic, Durrës Castle in Durrës, was constructed.

Anastasius was born at Dyrrhachium, the date is unknown, but he is thought to have been born no later than 430 or 431. He was born into an Illyrian family, the son of Pompeius (bornc. 410), nobleman of Dyrrachium, and wife Anastasia Constantina (bornc. 410). His mother was an Arian, sister of Clearchus, also an Arian, and a paternal granddaughter of Gallus (born c. 370), son of Anastasia (born c. 352) and husband, in turn daughter of Flavius Claudius Constantius Gallus and wife and cousin Constantina.

Anastasius had one eye black and one eye blue (heterochromia),and for that reason he was nicknamed Dicorus (Greek: Δίκορος, "two-pupiled").