Eutyches (378-c456), was a Byzantine monk and accidental heresiarch who rose from obscurity to give his name to one of the most famous heresies in the history of the Christian Church, Eutychianism, or the belief that the god and human aspects of Jesus Christ were fused into one single, hybrid man/god nature.

The archimandrite (or preacher) of a small congregation near Constantinople, Eutyches was fairly obscure and largely unknown in larger church circles until 448, when he was 70 years old and had already been leader of his small church for 30 years. In that year however, a synod was held at Constantinople, and Eutyches found himself caught up in a whirlwind of Church politics far beyond his control.

For decades a doctrinal battle had been raging between Nestorius, former Archbishop of Constantinople, and Cyril, Patriarch of Alexandria. Nestorius favored a position that Jesus Christ was born a mere human being, and only later took on a god-nature (a doctrine known as "Nestorianism"), whereas Cyril and his followers held a position that Christ always had dual god and human natures.

Seeking ideological allies on what was largely enemy turf, Cyril plucked Eutyches from his church and had him speak at the synod in favor of the Alexandrine position of two natures united in one person. The somewhat bumbling and overenthusiastic Eutyches, however, went too far however, in seeming to claim that rather than having two complete human and god natures united in one body, as the Alexandrines claimed, Jesus was actually more of a fusion of part god, part man. The shocked and outraged bishops at the synod thereupon voted to strip him of his priestly office, excommunicated him, and banished him from Constantinople.

The following year however, the Alexandrines struck back, when Cyril's successor as Patriarch of Alexandria Dioscorus succeeded in inducing Byzantine emperor Theodosius II to convene and ecumenical council, the Second Council of Ephesus, which was presided over Dioscorus himself. Drawing on the support of the massive contingent of monks he brought with him from Egypt, Dioscorus oversaw the reinstatement of Eutyches and the deposing of the bishops who had excommunicated him the previous year.

But Theodosius died the next year, and Empress Pulcheria and Emperor Marcian who succeeded him were much less sympathetic to the Alexandrine doctrine. Instead they favored a third option, championed by Leo, the Bishop of Rome (commemorated as "Pope Leo I"), which argued for two entirely distinct god and human natures inhabiting Jesus at the same time. Pulcheria and Marcian therefore summoned a new ecumenical council, the Council of Chalcedon, which repudiated the Council of Ephesus as a "robber synod" and annulled all its proceedings, thereby reinstating the excommunication of Eutyches, who died in exile and obscurity a few years later, at an unknown date.

Eutyches' doctrine of a fused, half-man, half-god Jesus Christ however, would later be supported by Byzantine empress Eudocia, and find a significant following in Syria. About a hundred years later, a Syrian bishop named Jacob Baradaeus would work tirelessly to unite all of the various persecuted groups of Syrian Eutychians, or "Monophysites" as they were also sometimes called, forming the Syriac Orthodox Church, which survives to the present day.