To the west of the south half of the Forum is a building that, because of its plan and location, can be identified as a basilica: a large, covered space that was used for economic transactions and as courthouse.

We know next to nothing about trials in Ostia, but a few remarks may be helpful. Judges belonged to the local elite. We do not find the designation iudex in the careers of the notables (cursus honorum), because this work was a public duty (munus). There were no professional judges. Imprisonment was an unknown form of punishment, it is a "modern invention". In antiquity prisons were used only to incarcerate suspects pending trial (such a prison has not been identified in Ostia, but is mentioned in the Christian martyrdom records). The punishment could amount to a fine, seizure, corporal punishment, forced labour, banishment, and the death penalty.

A basilica is characterized by several naves, created by long rows of columns. The central area was higher than the rest of the building, so that windows high up in the walls could provide light to the central area. This is how we should picture the Ostian Basilica.

The Basilica was excavated at least partially in the years 1802-1804. The masonry suggests a date in the period Domitian - Trajan (81-117 AD). The building was lavishly decorated with marble, most of which was robbed in recent times. Parts of columns and of the pavement remain. The building could be entered from the Decumanus Maximus via two lateral staircases, and from the Forum through a colonnade supporting arches. At the south end of the central nave are the remains of the podium used by the judges.

Fragments of marble reliefs that in all probability belong to the Basilica were found on various spots in the area to the west. Depicted are stories from the early history of Rome: the she-wolf suckling the twins; geese noticing enemies trying to enter the Capitol quietly, in 390 BC; the abduction of the Sabine women. Becatti has suggested that the reliefs formed part of a balustrade on the first upper floor in the interior of the Basilica. The reliefs and marble decoration of the colonnade along the Forum can be dated to the middle of the second century, on stylistic grounds, and were apparently added later to the building.

The Fasti Ostienses - municipal annals - mention the dedication of a basilica in Ostia in 152 AD. Becatti has argued that this is the Basilica at the Forum, but others maintain that the basilica of the Fasti should be looked for elsewhere in the city.