Byzantine (Eastern Roman) emperor whose reign saw the loss of Byzantium's southern and eastern provinces to the Arabs.

The son of the emperor Constantine III, Constans came to the throne in September 641, at age 11, after his father's death; during his minority the regency was under the control of the Senate of Constantinople. The Muslim Arabs seized Egypt from Byzantium in the second year of his reign and invaded Armenia in 647. In 655 he fought the naval battle of Phoenix (modern Finike, Turkey) off the coast of Asia Minor; his fleet was routed, and he escaped death only through the heroism of one of his soldiers. The murder of the caliph Uthmān ibn ʿAffānʿ in June 656 touched off a civil war among the Arabs that prevented them from attacking Constantinople, and in 659 Constans was able to secure a nonaggression treaty with the Arab governor of Syria. In 658 he successfully attacked the Slavs of Sclavinia and resettled captives in Asia Minor.

Constans's internal policy was marked by an attempt to force unity on the church, after theological disputes had divided the empire; in 648 he issued an edict, the Typos, forbidding argument about the controversial question of the divine and human natures of Christ. Pope Martin I condemned the Typos, and Constans, holding to the old conception of a single Roman Empire comprising East and West, had the pope arrested and exiled in 653. A similar punishment was meted out to the theologian Maximus the Confessor (Maximus the Confessor, Saint) in 655. (Both men are revered as saints in the Greek calendar.) The following year he made his son Constantine co-emperor, excluding his brother Theodosius from the succession and ordering his murder in 660.

Detested as a fratricide by his subjects in Constantinople, Constans in 663 left the capital and traveled westward, passing through northern Italy to Rome and then settling at Syracuse in Sicily. In 668 he alienated Pope Vitalian (Vitalian, Saint) by declaring Ravenna independent. His plans to make the city a permanent capital and a strategic centre for the defense of the West against the Arabs were cut short by his assassination.