Imperator Caesar Flavius Marcianus Augustus or Marcian (c. 390–January 457) was the emperor of the Byzantine Empire from 450 until his death.

Marcian was born in Thrace or Illyria. He spent his early life as an obscure soldier. He subsequently served for nineteen years under Ardaburius and Aspar, and took part in the wars against the Persians and Vandals. In 431, Marcian was taken prisoner by the Vandals in the fighting near Hippo Regius; brought before the Vandal king Geiseric, he was released on his oath never to take up arms against the Vandals.

Through the influence of these generals he became a captain of the guards, and was later raised to the rank of tribune and senator. On the death of Theodosius II he was chosen as consort by the latter's sister and successor, Pulcheria, and called upon to govern an empire greatly humbled and impoverished by the ravages of the Huns.

Upon becoming Emperor, Marcian repudiated the embarrassing payments of tribute to Attila the Hun, which the latter had been accustomed to receiving from Theodosius in order to refrain from attacks on the eastern empire. Aware that he could never capture the eastern capital of Constantinople, Attila turned to the west and waged his famous campaigns in Gaul 451 and Italy (452) while leaving Marcian's dominions alone.

He reformed the finances, checked extravagance, and repeopled the devastated districts. He repelled attacks upon Syria and Egypt (452), and quelled disturbances on the Armenian frontier (456). The other notable event of his reign is the Council of Chalcedon (451), in which Marcian endeavoured to mediate between the rival schools of theology.

Marcian generally ignored the affairs of the western Roman Empire, leaving that tottering half of the empire to its fate. He did nothing to aid the west during Attila's campaigns, and, living up to his promise, ignored the depredations of Geiseric even when the Vandals sacked Rome in 455.

Shortly before Attila's death in 453, conflict had begun again between him and Marcian. However, the powerful Hun king died before all-out war broke out. In a dream, Marcian claimed he saw Attila's bow broken before him, and a few days later, he got word that his great enemy was dead.

Marcian died in 457 of disease, possibly gangrene contracted during a long religious journey.

Despite his short reign and his writing off of the west Marcian is considered one of the best of the early "Byzantine" emperors. The Eastern Orthodox Church recognizes him and his wife Pulcheria as saints, with their feast day on February 17.

Another but lesser Marcian was a son-in-law of Byzantine Emperor Leo I and his queen Verina. He aided Verina in the clash against another son-in-law, Emperor Zeno.