The baptistery is in the northern part of the church, has an octagonal central part with a corridor surrounding it. To the east and west were halls with apses for the pre- and post-baptism ceremonies. The whole dates from before the Justinian basilica was built and was used when the basilica had been erected. A walk in-walk out baptismal pool is at the heart of the centre.

Looking to the centre, from what seems to have been the entrance to the surrounding corridor. The main body of the basilica is to the right.