The Baptistery is the older of the two churches in the medieval borgo of St. Severa and is positioned close to the original entrance.
The small chapel holds a baptismal font made from a Roman millstone and has frescoes from the second half of the 15th century, traceable to a culture typical of the Lazio area, with archaic aspects, which is extremely reminiscent of the youthful works by Antoniazzo Romano.
The frescoes on the apse feature a central Enthroned Madonna and Child, flanked on the left by St. Sebastian pierced by spears and St. Severa who is indicating the Virgin to a Member of the Confraternity of the Holy Spirit, recognisable by the double cross on his clothing who could well be the commissioner of the frescoes. To the right are St. Lucia and St. Roche.
Three medallions on the transverse arches with the heads of Abraham, David and Daniele.
On the end wall, above the apse, a scene of the Annunciation, with the Christ in a central oval, has been cut off by the modern ceiling.
St. Anthony Abbot is standing in the lower left portion, while the painted surface to the lower right has disappeared.
The left wall has been painted with the scene of a probable miraculous catch of fish, with two Bishop Saints appearing to the members of a ship's crew which bears the symbol of the Confraternity of the Holy Spirit on the white pennant flying from the mast. There is a sailor on the mast, below the maintop. There is a small boat with two men in it under the ship's stern, one of them holding the ship's cable. St. Lawrence is standing in front of the stern, with a gridiron in his left hand and the Rule in his right. The remains of a very damaged painting on the right wall shows part of a Crucifixion with a kneeling Mary Magdalene and a standing, hand clasped Madonna. The preparation cartoons for the painting are still visibile etched into the plaster base.
The draped base is decorated with interesting votive drawings of 15th and 16th century ships, probably drawn by the sailors who travelled to the harbour of St. Severa throughout the centuries. These drawings are something of an archive of naval iconography for the era.