The Second Council of Ephesus was a Christological church synod in 449 convened by Emperor Theodosius II under the presidency of Pope Dioscorus I of Alexandria. It was intended to be an ecumenical council, and it is accepted by the miaphysite churches, who view it as a valid continuation of the First Council of Ephesus if not an ecumenical council in its own right. The Second Council of Ephesus was explicitly repudiated by the next council, the Council of Chalcedon of 451. The Council of Chalcedon is recognized as the fourth ecumenical council by Chalcedonian Christians, and the Second Council of Ephesus was named the Latrocinium ("Robber Synod") by Pope Leo I; the Chalcedonian churches, particularly the Roman Catholic and Eastern Orthodox communions, continue to accept this designation, while the Oriental Orthodox repudiate it.

Both this council and that at Chalcedon dealt primarily with Christology, the study of the nature of Christ. Both councils affirmed the doctrine of the hypostatic union and upheld the orthodox Christian doctrine that Jesus Christ is both fully God and fully man. The Second Council of Ephesus decreed the formula of Cyril of Alexandria, stating that Christ is one incarnate nature [mia physis] (a qualitative description of the union of divinity and humanity), fully human and fully God, united without separation, without confusion, without mixture and without alteration. The Council of Chalcedon decreed that in Christ two natures exist, "a divine nature [physis] and a human nature [physis], united in one person [hypostasis], with neither division nor confusion".

Those who do not accept the decrees of Chalcedon nor later ecumenical councils are variously named monophysites, miaphysites, or non-Chalcedonians, and comprise what is today known as Oriental Orthodoxy, a communion of six autocephalous ecclesial communions: Coptic Orthodox Church of Alexandria, Ethiopian Orthodox Tewahedo Church, Eritrean Orthodox Tewahedo Church, Malankara Orthodox Syrian Church, Syriac Orthodox Church, and the Armenian Apostolic Church. Those who accepted the teaching of Chalcedon but resided in areas dominated by Oriental Orthodox bishops were called by the non-Chalcedonians Melkites ("King's men"), as the Emperors were usually Chalcedonians. The Antiochian Greek Orthodox Church and Melkite Greek Catholic Church historically descend from these people. Shortly after the Council of Chalcedon, Pope Dioscorus was deposed by the Council of Chalcedon but still recognized by Miaphysites until his death in 454 followed by Pope of Alexandria for the miaphysite party in opposition to the Chalcedonian Pope of Alexandria. Over the next few centuries, various popes usually held to either one side or the other although some accepting the Henotikon. Eventually, two separate papacies were established, each claiming sole legitimacy.

Nestorius was Archbishop of Constantinople. His opponents charged him with detaching Christ's divinity and humanity into two persons existing in one body, thereby denying the reality of the Incarnation. It is not clear whether Nestorius actually taught that. A combination of politics and personalities contributed to Nestorius being judged a heretic and deposed at the Council of Ephesus in 431 AD. John Anthony McGuckin sees an "innate rivalry" between the Sees of Alexandria and Constantinople.

Eutyches was an archmandrite in Constantinople. In his opposition to Nestorianism, he seemed to take an equally extreme, although opposite view. In 448, Flavian, Bishop of Constantinople held a synod at which Eusebius, Bishop of Dorylaeum, brought a charge of heresy against Eutyches. Eutyches was summoned to appear and clarify his position regarding the nature of Christ. Finding his response unsatisfactory, the synod condemned and exiled Eutyches, who sent an appeal to Pope Leo I. When Leo had received the Acts of the council, he concluded that Eutyches was a foolish old man who had erred through ignorance, and might be restored if he repented. Dioscurus of Alexandria, imitating his predecessors in assuming a primacy over Constantinople, simply annulled the sentence of Flavian, and absolved Eutyches. Dioscurus and Eutyches had obtained the convocation by the Emperor of an ecumenical council to meet at Ephesus in August, 449.^{[citation needed]}

The Acts by the Second Council of Ephesus are known through a Syriac translation by a monk, that was published by the British Museum (MS. Addit. 14,530) and written in 535. The first session is missing.

There was insufficient time for Western bishops to attend except a certain Julius, Bishop of Puteoli, who, together with a Roman priest, Renatus (who died on the way), and the deacon Hilarius (who later became Pope himself), represented Pope Leo I. The emperor gave Dioscorus of Alexandria the presidency: ten authentian kai ta proteia (Greek). The legate Julius is mentioned next, but when his name was read at Chalcedon, the bishops cried: "He was cast out; no one represented Leo". Next in order is Juvenal of Jerusalem, above both the Patriarch Domnus II of Antioch and Patriarch Flavian of Constantinople.

There were 127 bishops present at the council, with eight representatives of absent bishops, and lastly the deacon Hilarius with his notary, Dulcitius. The question before the council, by order of the emperor, was whether Archbishop Flavian of Constantinople, in a synod held by him at Constantinople beginning on 8 November 448 AD, had deposed and excommunicated Archimandrite Eutyches for refusing to admit two natures in Christ. Consequently, Flavian and six other bishops, who had been present at his synod, were not allowed to sit as judges in the council.