Alexander I of Alexandria (Koine Greek: Ἀλέξανδρος, Aléxandros) was the 19th Patriarch of Alexandria. During his patriarchate, he dealt with a number of issues facing the Church in that day. These included the dating of Easter, the actions of Meletius of Lycopolis, and the issue of greatest substance, Arianism. He was the leader of the opposition to Arianism at the First Council of Nicaea. He also mentored his successor, Athanasius of Alexandria, who would become one of the Church Fathers. He is regarded as a Pope by the Coptic Orthodox Church but not by the Catholic Church.

Comparatively little is known about Alexander's early years. During his time as a priest, he experienced the bloody persecutions of Christians by Emperors Galerius and Maximinus Daia.

Alexander became patriarch on the passing of Achillas of Alexandria, whose own remarkably short reign was thought by some to have been brought about by his breaking the command of his own predecessor, Peter of Alexandria, to never readmit Arius into communion.

Alexander himself faced three primary challenges during his patriarch term. The first of these was a schismatic sect, led by Erescentius, which disputed the timing of Easter. Alexander found himself put in the position of writing a special treatise on the controversy, in which he cited earlier statements regarding the matter by Dionysius of Alexandria. Alexander's own efforts, while they did serve to quiet the dispute, were not enough to quiet the controversy themselves, although the First Council of Nicaea, held during his tenure, did resolve the matter.

His second major concern was the matter of Meletius of Lycopolis, who continued to slander Alexander, as he had earlier done to Achillas. Meletius went so far as to lodge a formal complaint with the court of the Emperor Constantine I, although no unusual attention was given it.

More important, however, was that Meletius had seemed to establish some form of working alliance with Arius. Meletius also consecrated bishops of his own, without his superior's consent. This controversy would continue unabated until the Council at Nicaea, where Alexander allowed Meletius to return to the church, effectively ending Meletius' alliance with Arius.

The last, and most important, of the problems Alexander faced was the issue of Arius himself. Alexander's predecessor, Achillas, had not only allowed Arius to return to the church, but had given him the oldest church in Alexandria, a position which allowed him to exercise a great influence on the Christian community of Alexandria. In fact, Arius was even a contender for the post of patriarch of Alexandria at the death of Achillas.

The conflict between the two began in earnest when Alexander declared the unity of the Trinity in one of his sermons. Arius immediately responded by labeling Alexander's statement Sabellianism, which had already been rejected by that time. The controversy quickly escalated, and Arius developed ever increasing support for his position, winning over a number of deacons, and at least one presbyter, who started to ordain presbyters of his own. Arius continued to draw even more attention and support, to the point that Alexander found himself having to summon two separate assemblies of his priests and deacons to discuss the matter. Neither of these assemblies, though, reached any firm conclusions, or helped to limit the spread of Arius' beliefs.