The Forum, the main square of the city, was remodeled extensively during the reign of Hadrian (117-138 AD). Most of the Forum has been excavated further, to explore the early phases of Ostia. These excavations will not be discussed here, however.

From the period of Hadrian the square was surrounded by four major public buildings: the Capitolium at the north end, the Temple of Rome and Augustus at the south end, the Curia towards the north-west, and the Basilica towards the south-west. From the north and south only pedestrians could enter the square. The Decumanus Maximus on the other hand continues across the square. To the south of this road is an enigmatic, small, round structure.

The square was covered with a layer of concrete, but it is not known what finishing stone was on top. On the west and east side are Hadrianic porticoes with granite columns and marble pavement, three steps above the Forum. From the north-east portico three rooms and a staircase can be accessed, from the south-east portico a niche with a statue (I,XII,11). To the west of the Temple of Rome and Augustus is a row of shops (I,XI,4).

The Forum was probably adorned with a statue of Ancus Marcius, king of Rome (640-616 BC), and according to many ancient authors founder of the colony of Ostia. The inscription belonging to this statue was found in a sewer below the Via dei Molini (CIL XIV, 4338; Cebeillac-Gervasoni M. - Caldelli M.L. - Zevi F. 2006, inscription # 1):

Statues of the kings had also been erected in Rome, on the Forum Romanum and the Capitoline Hill. The Ostian inscription may well belong to the later second century AD, a period of antiquarian writings. Note that Ostia is called the first, oldest colony of Roman citizens.

Various statues of notables were placed on the Forum, for example an equestrian statue of C. Domitius Fabius Hermogenes, priest of the deified Hadrian, and an equestrian statue of Manilius Rusticus, Prefect of the Annona (grain supply) and patron of the city, at the end of the fourth century. An inscription from the same period tells us about the transfer of a statue to the Forum ex sordentibus locis, "from a sordid place".

Presumably near the Forum was an altar of Concordia, used during wedding ceremonies. It was erected after the death of Faustina, wife of Antoninus Pius, in 140 AD. Near the altar was an inscription on a slab, that was reused in the Terme del Foro (CIL XIV, 5326). We read that the altar was erected "utique in ara virgines quae in colonia Ostiensi nubent, item mariti earum, supplicent" ("so that on this altar the young girls who marry in the colony of Ostia, like their husbands, can bring sacrifices").