Ostia, Basilica Constantiniana was the 4th century cathedral church of the ancient town of Ostia, only surviving as scanty foundations.
The dedication has been given in scholarly publications as SS Peter, Paul and John the Baptist. This depends on the rather forced conflation of several sources, and an alternative argument is that the dedication was to St Laurence.
A certain Gallicanus Avinius, a retired consul and army commander living at Ostia, is described as making a donation towards the cost. However a later legend ascribed to him the entire responsibility for the project and listed him as a martyr in Alexandria. This is a pious 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 of the diocese of Ostia.
Evidence was found by the archaeologists 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.
The site was then forgotten, to the extent that there used to be debate as to whether Sant'Aurea a Ostia Antica had always been the cathedral of the diocese. However, 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, but sufficient to establish a plan.
The cathedral 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. It is outside the visitable area of the Ostia ruins.
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 central nave with aisles and arcades having fourteen columns each as well as 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.
The foundations were re-buried, and there is nothing to see.