Ostia, Basilica Constantiniana was the cathedral church of the ancient town of Ostia from about the years 330 to after 700. It was then replaced by Sant'Aurea a Ostia Antica . It was in the south-east quarter of the town, just by the gate in the city walls at the end of Via del Sabazeo and to the east of the street.
The "Liber Pontificalis" for the reign of Pope Silvester (314-335) describes how the emperor Constantine arranged for the erection of a basilica in Ostia, together with a baptistery. The dedication was to SS Peter, Paul and John the Baptist. A certain Gallicanus Avinius, a retired consul and army commander living at Ostia, is described as making a donation towards the cost. (However, the "Acta Sanctorum" ascribed to him the entire responsibility for the project and listed him as a martyr in Alexandria, a fiction which has been rejected by the revised Roman martyrology.)
Although no contemporary sources confirm this, it is almost certain that the church became the cathedral. After it was abandoned in the early 9th century, the site was forgotten. There used to be debate as to whether Sant'Aurea had always been the cathedral of Ostia, a traditional view, but in 1996 a geophysical survey was made of the site by the German Archaeological Institute at Rome and the buried foundations were revealed. Several sondage trenches were dug in 1998 and the following year to confirm the identification and to try and establish a chronology. The remains of the walls even in the foundations were scanty, and there is nothing on view now.
The church was approached from the street firstly via a small piazza and then by an enclosed courtyard or atrium, the equivalent of a mediaeval monastic cloister with walkways on four sides. The edifice itself had a nave with aisles and arcades having fourteen columns each and a semicircular apse, being just over 51m in length and 23m in width. A separate baptistry was added after the main structure was completed, and this was on the right side of the atrium. It was rectangular, with an unusual apse of almost a three-quarter circle. Evidence was found that the complex was in decay and being misused in part for domestic purposes throughout the sixth century. It was abandoned and plundered for moveable salvage in the eighth century, and around the year 800 the remaining walls finally collapsed. Almost all of the stonework was then removed for use elsewhere.