The octagonal design of the building, employed in virtually all Early Christian baptisteries, symbolizes the seven days of the week plus the Day of the Resurrection and Eternal Life. The octagonal design extends to the font as well.

It is called the Orthodox Baptistry to distinguish it from the Arian Baptistry constructed on behest of Ostrogothic king Theodoric some 50 years later. This octagonal structure was built ca. 400-450 as a baptistery for the cathedral and is similar to the baptistery of St. Tecla in Milan. Bishop Neon (reigned 449-458) added the dome and the upper part of the outer walls ca. 450; the former is of terracotta tubes, and the latter is articulated with blind arcades and pilasters, as is the exterior of the mausoleum of Galla Placidia. The original floor is more than 3 m below the present exterior ground level. Neon also decorated the interior with mosaics. It is the most ancient monument remaining in Ravenna, one of eight Ravenna sites on the UNESCO World Heritage List.