Founded in 138 BC, under the consulate of Decimus Junius Brutus, Valencia was a settlement for regular soldiers who were also granted land near the new city. Archaeological excavations have unearthed evidence of the first settlement such as holes used to support posts for log cabins and tents at what was most probably a makeshift camp which, within a few years, had given way to more permanent constructions. Valentia quickly prospered and before long started to coin its own money.

The city was razed to the ground in 75 BC during the war waged between Pompey and Sertorius. Discoveries at a site at La Almoina include the dismembered bodies of various soldiers alongside their weapons, which indicates that a skirmish occurred there. As a result of the conflict, it appears that the settlement was virtually abandoned for at least half a century.

From the middle of the first century onwards Valentia gradually recovered lost ground and embarked on a lengthy period of development typified by urban growth, the wealth of the new settlers and the aggrandisement of the city through the construction of large public buildings, such as the forum and the amphitheatre. Sizeable infrastructure projects were also completed, such as a river port next to the modern-day Torres dels Serrans or the water supply system, a facility that the people of Valencia were unable to make use of again until the middle of the nineteenth century.

Valentia was not immune to the crisis that afflicted the rest of the Roman Empire in the second half of the third century and the city entered a lengthy period of decline during which its boundaries shrank, whole suburbs were left deserted and its infrastructures abandoned. In the middle of the fourth century the city was inhabited by a Christian community drawn there in memory of San Vicente, who was martyred in Valencia in 304.

A century later, at the same time as the first waves of Germanic invaders arrived and in response the power vacuum bequeathed by the Romans, the church took the reins of the city, replacing ancient Roman temples with Christian places of worship. In the sixth century, during the time of Bishop Justinian, Valentia recovered its former glory to some extent. Urban decline was momentarily halted and the city played host to an important regional council. The city acquired a strategic importance following the Byzantine invasion of the south east of the Iberian peninsula in 554 with Visigoth armies using it as a base and fortifying the ancient Roman amphitheatre. The expulsion of the Byzantines in 625 heralded a dark period and what little documentation there is suggests that the city developed little during this period.