According to tradition, this Byzantine church is a 9th-century-A.D. construction that housed the holy relics of St. Theodosia, an 8th-century-A.D. nun who was tortured and then martyred defending a sacred icon of Christ set into Constantinople's bronze gate. As the resting place for the saint's bones, the church became a point of pilgrimage, as the relics were believed to have healing powers.

In fact, the church is alternately thought to be that of Saint Euphemia, Saint Theodosia, and Eurgetis Monastery, and indeed the entire affair dates only to the 12th century. To complicate the legend, Catholic sources question the entire validity of St. Theodosia's martyrdom.

The cellar of the church was used as a shipyard warehouse immediately after the conquest and was converted into a mosque in the 16th century. The interior is a bit whitewashed and dreary. A tomb to the right of the mimbar is inscribed as "One of Jesus's Companions, the Apostle"; one tradition places the tomb of Constantine XI Dragases, last Emperor of Byzantium, here, while locals believe that it is the tomb of "Gül Baba."