Dioscorus I (Greek: Διόσκορος Α΄ ὁ Ἀλεξανδρείας), also known as Dioscorus the Great, was the pope of Alexandria and patriarch of the See of St. Mark who was deposed by the Council of Chalcedon in 451. He was recognized as patriarch by the Coptic Church until his death. He died in Gangra, Paphlagonia, in September 454. He is venerated as a saint by the Coptic and other Oriental Orthodox Churches.

Dioscorus' origins are unknown, although it is known that he was not a Copt. Dioscorus served as the dean of the Catechetical School of Alexandria, and was the personal secretary of Cyril of Alexandria, whom he accompanied to the Council of Ephesus in 431. He eventually rose to the position of archdeacon. He had been made Cyril's designated successor.

In his struggle against Nestorius, Cyril explained the union between the divine and human natures of Christ as "inward and real without any division, change, or confusion." He rejected the Antiochene theory of "indwelling", or "conjunction" or "close participation", as insufficient. Thus the Alexandrian formula adopted by Cyril and Dioscorus was, in Greek, mia physis tou theou logou sesarkomene, which translates into "one nature of God the Word Incarnate", by which Cyril meant "one nature"—that Christ is at once God and man. On the other hand, the Antiochene formula was "two natures after the union", or "in two natures", which translates to dyo physis. This formula explained Christ as existing in two natures, God and man.^{[citation needed]}

Nestorius was condemned and deposed by the First Council of Ephesus, which approved of the Second Epistle of Cyril to Nestorius.

Dioscorus succeeded Cyril as Patriarch of Alexandria in the summer of 444.

Soon after Dioscorus' consecration, Theodoret sent a letter (Letter 60) to him. In this letter he praises Dioscorus' modesty and humility, stating: "you do not behold the multitude of your subjects nor the exaltation of your throne, but you see rather human nature, and life's rapid changes, and follow the divine laws whose observance gives us the kingdom of heaven". Though being theologically opposed to him, Theodoret always positively represents Dioscorus' qualities. Theodoret had written against the 12 Anathemas of Saint Cyril and did not condemn Nestorius until the Council of Chalcedon. W. Bright states that Theodoret's: "testimony in his favour cannot be suspected".

Cyril's nephew Athanasius, who C. Haas describes as a "wealthy Alexandrian priest and potential rival" was a fierce critic of Dioscorus. Immediately after his accession Dioscorus was worried that the influential group of Cyril's relatives would be tempted to sabotage his tenure. He was also legally obliged to persecute them for misappropriating church property. Dioscorus regained property (including boats) for the Church, and destroyed property and fruit trees that had been illegally situated on land belonging to the Church. Athanasius claimed that from the outset of his episcopate, Dioscorus harassed him and his brother by using influence with the court, so that the brother died of distress, and Athanasius, with his aunts, sister-in-law, and nephews, were bereft of their homes by the patriarch's malignity. According to the deacon Ischyrion in the third session of the Council of Chalcedon in 451, Dioscorus had laid waste to property, inflicted fines and exile, as well as buying up and selling at a high price the wheat sent by the government to Libya. In Patriarch Dioscorus of Alexandria: The Last Pharaoh and Ecclesiastical Politics in the Later Roman Empire, V.L. Menze writes that illegal property had been "bearing income to the deacon Ischyrion".

Eutyches was an archimandrite in Constantinople. In his opposition to Nestorianism, he seemed to take an equally extreme, although opposite view. Eutyches claimed to be a faithful follower of Cyril. In November 448, Flavian, Bishop of Constantinople held a synod regarding a point of discipline connected with the province of Sardis. Eutyches had been accusing various personages of covert Nestorianism, and at the end of the session of this synod one of those inculpated, Eusebius, Bishop of Dorylaeum, brought the question forward, and proffered a counter charge of heresy against the archimandrite. Eutyches was summoned to clarify his position regarding the nature of Christ. When asked whether Christ was in two natures after the union, Eutyches responded by stating: "I confess that our Lord was of two natures before the union, but after the union, one nature... I follow the teaching of the blessed Cyril and the holy Fathers and the holy Athanasius, because they speak of two natures before the union, but after the union and incarnation, they speak not of two natures but of one nature." Flavian had demanded that Eutyches confess two natures. Finding his response unsatisfactory, the synod condemned and exiled Eutyches as well as excommunicating anyone associated with him.