The town has its roots in prehistoric times, when the first settlements appeared in the area probably already in relation to the use of the precious thermal waters, whose therapeutic virtues must have been considered miraculous at the time. From this main attraction of the territory derives the first name, from the Latin era, which has come down to us, namely that of Aquae Hypsitanae and, already in pre-Roman times there was a sanctuary of the “domus de janas” type dedicated to the cult of the waters.
In the course of its history it has been to represent first for the Carthaginians and then for the Romans an important military and defensive center, both for its happy position in the center of the island, almost exactly halfway between Turris Libisonis (Porto Torres) and Caralis (Cagliari), which due to the presence of a particularly easy ford of the Tirso river
At the time of the Emperor Trajan, in the period around 100 AD, the spa changed its name and qualification, becoming Forum Traiani (hence the current name), a sort of “free city” that served as a peaceful place of exchange and market between the Romans settled on the island and the indigenous populations of Barbaria.
With the empire of Trajan begins a continuous rise that leads Fordongianus to be one of the richest and most powerful cities on the whole island: around the third century AD. it was first raised to the rank of Municipium, then it became a bishopric; finally, under the Emperor Justinian, in the fifth century AD, the city was promoted to the rank of oppidum and fortified with a wall, as the seat of the Dux Sardiniae, that is, the military commander of the island and the seat of the local treasury.