The essay focuses on the examination of the hagiographic testimonies related to the cult of saint Euphemia of Chalcedon, questioning its cultic origin and highlighting the fundamental role of the Council of 451. The identification of a series of sources, often unreliable and discordant, complicates the reconstruction of the original martyrdom of the saint. It is a varied corpus that, however, is overlooked by the subsequent early medieval tradition, which identifies the passio written following the Council of Chalcedon as the true story of the martyr. The article emphasizes the different narratives of the martyrdom, clearly adapted to the needs of the communities that, from the 4th century onward, knew the veneration of Euphemia. This textual modification process, typical of the late antiquity and early middle ages, in the case of the saint of Chalcedon, appears emblematic and capable of ensuring the widespread diffusion of the cult in the major centers of christian Europe.