Eusebius of Nicomedia and Constantinople, (d. 341) was a bishop of Berytus (modern-day Beirut) in Phoenicia, then of Nicomedia where the imperial court resided, and finally of Constantinople from 338 up to his death.

Distantly related to the imperial family of Constantine, he not only owed his removal from an insignificant to the most important episcopal see to his influence at court, but the great power he wielded in the Church was derived from that source. With the exception of a short period of eclipse, he enjoyed the complete confidence both of Constantine and Constantius II; and it was he who baptized the former May, 337.

Like Arius, he was a pupil of Lucian of Antioch, and it is probable that he held the same views as Arius from the very beginning. He afterward modified his ideas somewhat, or perhaps he only yielded to the pressure of circumstances; but he was, if not the teacher, at all events the leader and organizer, of the Arian party.

At the First Council of Nicaea, 325, he signed the Confession, but only after a long and desperate opposition. His defense of Arius angered the emperor, and a few months after the council he was sent into exile. After the lapse of three years, he succeeded in regaining the imperial favor; and after his return in 329 he brought the whole machinery of the state government into action in order to impose his views upon the Church.

He is not to be confused with his contemporary Eusebius of Caesarea, the author of a well-known early book of Church History.