The First Council of Nicaea (; Ancient Greek: Νίκαια [ˈnikεa]) was a council of Christian bishops convened in the Bithynian city of Nicaea (now İznik, Turkey) by the Roman Emperor Constantine I in AD 325.

This ecumenical council was the first effort to attain consensus in the church through an assembly representing all Christendom. Hosius of Corduba may have presided over its deliberations.Its main accomplishments were settlement of the Christological issue of the divine nature of God the Son and his relationship to God the Father, the construction of the first part of the Nicene Creed, mandating uniform observance of the date of Easter, and promulgation of early canon law.