St. Euphemia was born in Chalcedon (present-day Kadköy), the daughter of devout parents, Philophron and Theodosiani. She was tortured during the persecutions of Emperors Diocletian and Maximian in the late third century.

The Saint played a major role in inspiring the Fathers of the Fourth Ecumenical Council. During that council (451), St. Euphemia worked a miracle that determined the final doctrinal definition. The 630 Fathers, who gathered for this council in Chalcedon, were deliberating about the two natures of Christ. Eutyches and Dioscoros claimed that Christ possessed only a single nature. To test this teaching, the Holy Fathers inscribed the differing opinions on two separate decrees, which they placed inside the reliquary of St. Euphemia. When the reliquary was later opened, the decree of the heretics had fallen to the feet of the Saint, while the Orthodox doctrine rested in her hands. The Orthodox Church celebrates this miracle on July 11. The repose of St. Euphemia is commemorated on September 16.