Alexander I of Alexandria (died 326) was a Bishop of Alexandria and the leader of the opposition to the Arian heresy at the First Council of Nicaea. He was also the mentor of Athanasius of Alexandria, who would become one of the leading Church fathers and most prominent opponent of Arianism.

In 321, Alexander called a general council of the entire church of the nation to deal with Arius’ teachings. The council gathered no fewer than one hundred participants. At this council, Arius continued to argue that the Son could not be co-eternal with the father, and even went on to say that the Son was not similar to the Father in substance. The assembled council then placed Arius under anathema until he recanted his positions.  Arius’ followers in Alexandria began to engage in violence in defense of their beliefs and threatened to damage the peace and unity of the church and of the empire.  Emperor Constantine sent a letter requesting that Alexander and Arius end their dispute.

Arius himself formally complained to the emperor over his treatment by Alexander. In response, Constantine called for Arius to plead his case before an ecumenical council of the church, to be held at Nicaea. Alexander was himself supposed to preside over the First Council of Nicaea, but felt that he could not serve as both presiding official and chief accuser. On that basis, he turned over the presidency to Hosius of Cordova.  The Council decidedly ruled against Arius. Five months after returning to Alexandria from Nicaea, Alexander died.  He is considered the 19th Pope and Patriarch of Alexandria by the Coptic Orthodox Church.