Anastasius I Dicorus (Ancient Greek: Ἀναστάσιος, romanized: Anastásios; c. 431 – 9 July 518) was Roman emperor from 491 to 518. A career civil servant, he came to the throne at the age of 61 after being chosen by Ariadne, the wife of his predecessor, Zeno. His reign was characterized by reforms and improvements in the empire's government, finances, economy and bureaucracy. The resulting stable government, reinvigorated monetary economy and sizeable budget surplus allowed the empire to pursue more ambitious policies under his successors, most notably Justinian I. Since many of Anastasius' reforms proved long-lasting, his influence over the empire endured for centuries.

Anastasius was a Monophysite Christian and his personal religious tendencies caused tensions throughout his reign in the empire that was becoming increasingly divided along religious lines.

Anastasius was born at Dyrrachium; the date is unknown, but is thought to have been no later than 431. He was born into an Illyro-Roman family. Anastasius had one black eye and one blue eye (heterochromia), and for that reason he was nicknamed Dicorus (Ancient Greek: Δίκορος, romanized: Díkoros, "two-pupiled"). Before becoming emperor, Anastasius was a silentiary.

Anastasius is known to have had a brother named Paulus, who served as consul in 496. With a woman known as Magna, Paulus was father to Irene, who married Olybrius. This Olybrius was the son of Anicia Juliana and Areobindus Dagalaifus Areobindus. The daughter of Olybrius and Irene was named Proba. She married Probus and was mother to a younger Juliana. This younger Juliana married her possible relative another Anastasius, son of Anastasius and maternal grandson of Theodora, and was mother of Areobindus, Placidia, and a younger Proba, who married (Flavius) Anastasius, born in 530, and mothered Areobindus, born in 550, and Placidia, born in 552 and wife of John Mystacon. Another nephew of Anastasius was Probus, consul in 502. Anastasius' sister, Caesaria, married Secundinus, and gave birth to Hypatius and Pompeius. Anastasius Paulus Probus Moschianus Probus Magnus, consul in 518, was a great-nephew of Anastasius. His daughter Juliana later married Marcellus, a brother of Justin II. The extensive family may well have included several viable candidates for the throne.

In the weeks following the death of Zeno (491), crowds gathered in the Hippodrome of Constantinople and demanded a Chalcedonian and properly Roman successor, chanting "Give the Empire an Orthodox emperor! Give the Empire a Roman Emperor!" Under such pressure, Ariadne, Zeno's widow, turned to Anastasius. Anastasius was in his sixties at the time of his ascension to the throne. Ariadne chose Anastasius over Zeno's brother Longinus, which upset the Isaurians. Once he took office, Anastasius exiled Longinus and purged a number of Isaurians officials from government. Religiously, Anastasius' sympathies were with the Monophysites. Consequently, as a condition of his accession, the patriarch of Constantinople required that he pledge not to repudiate the Council of Chalcedon.

Ariadne married Anastasius on 20 May 491, shortly after his accession on 11 April. He gained popular favour by a judicious remission of taxation, in particular by abolishing the hated tax on receipts, which was mostly paid by the poor. He displayed great vigour and energy in administering the affairs of the empire. His reforms improved the empire's tax base and pulled it from financial depression and bleak morale. By the end of his reign, it is claimed that the treasury had 320,000 lb gold reserve.

Not long after his accession, the circus factions caused riots and set fires around the Hippodrome. Though its exact cause is unclear, the riot was pacified when Anastasius replaced the city prefect Julian with his brother-in-law Secundinus.

Under Anastasius, the empire engaged in the Isaurian War against Longinus and the Anastasian War against Sassanid Persia.