Polycrates of Ephesus was a bishop in the late 2nd century CE, during the Roman era. He is a significant figure in early Christian history due to his role in a major liturgical dispute known as the Quartodeciman controversy. This debate centered on whether to celebrate Easter on the 14th day of the Jewish month of Nisan (the date of Passover) or on the following Sunday.

Polycrates was a leading defender of the traditional Asian practice of observing Easter on 14 Nisan. His main documented action was writing a letter around 190 CE to Pope Victor I of Rome, arguing against attempts to excommunicate churches that followed this custom. In the letter, he appealed to local apostolic tradition, citing figures like the Apostle John and Polycarp of Smyrna, and noted his own long family lineage of bishops. According to church historians, this confrontation highlighted early tensions between regional traditions and the growing central authority of Rome. The immediate crisis was resolved without a formal schism, but the Sunday observance for Easter eventually became standard.

Only one of his works survives: his Letter to Pope Victor and the Bishops. It is preserved not independently, but within the Ecclesiastical History written by Eusebius in the 4th century. Modern scholars view Polycrates as an important witness to the diversity of practice and the strength of regional identity in early Christianity before it became a unified imperial religion.