Flavius Anastasius or Anastasius I (c. 430–July 9, 518) was Byzantine emperor from 491 until his death. He was born at Dyrrhachium not later than A.D. 430. At the time of the death of Zeno (491), Anastasius, a palace official (silentiarius ), held a very high character, and was raised to the throne of the Roman empire of the East, through the choice of Ariadne, Zeno's widow, who married him shortly after his accession. His reign, though afterwards disturbed by foreign and internecine wars and religious distractions, commenced auspiciously. He gained the popular favour by a judicious remission of taxation, and displayed great vigour and energy in administering the affairs of the empire.

The principal wars in which Anastasius was engaged were those known as the Isaurian and the Persian. The former (492-496) was stirred up by the supporters of Longinus, the brother of Zeno. The victory of Cotyaeum in 493 "broke the back" of the revolt, but guerrilla warfare continued in the Isaurian mountains for some years longer. In the war with Persia (502-505), Theodosiopolis and Amida were captured by the enemy, but the Persian provinces also suffered severely and the Romans recovered Amida. Both adversaries were exhausted when peace was made (506) on the basis of status quo. Anastasius afterwards built the strong fortress of Daras to hold Nisibis in check. The Balkan provinces were devastated by invasions of Slavs and Bulgarians; to protect Constantinople and its vicinity against them he built the Anastasian wall , extending from the Propontis to the Euxine.

The emperor was a convinced Monophysite, but his ecclesiastical policy was moderate; he endeavoured to maintain the principle of the Henotikon of Zeno and the peace of the church. It was the uncompromising attitude of the orthodox extremists, and the rebellious demonstrations of the Byzantine populace, that drove him in 512 to abandon this policy and adopt a monophysitic programme. His consequent unpopularity in the European provinces was utilized by an ambitious man, named Vitalian, to organize a dangerous rebellion, in which he was assisted by a horde of "Huns" (514-515); it was finally suppressed by a naval victory won by the general Marinus. The financial policy of Anastasius was so prudent and economical that it gained him a reputation for avarice and contributed to his unpopularity.

There is a story about his choosing of a successor: Anastasius could not decide which of his three nephews should succeed him, so he put a message under a couch and had his nephews take seats in the room, which also had two other seats; he believed that the nephew to sit on the special couch would be his proper heir. However, two of his nephews sat on the same couch (one story has it that they were incestuous lovers), and the one with the concealed message remained empty. Then, after putting the matter to God in prayer, he determined that the first person to enter his room the next morning should be the next emperor, and that person was Justin, the chief of his guards. In fact, Anastasius probably never thought of Justin as a successor, but the issue was decided for him after his death.