Romanization began with the arrival of the Roman army in the Iberian Peninsula in 218 BCE during the Second Punic War against Carthage. The Romans sought to conquer Lusitania, a territory that included all of modern Portugal south of the Douro river and Spanish Extremadura, with its capital at Emerita Augusta (now Mérida).

Mining was the primary factor that made the Romans interested in conquering the region: one of Rome's strategic objectives was to cut off Carthaginian access to the Iberian copper, tin, gold, and silver mines. The Romans intensely exploited the Aljustrel (Vipasca) and Santo Domingo mines in the Iberian Pyrite Belt which extends to Seville.

While the south of what is now Portugal was relatively easily occupied by the Romans, the conquest of the north was achieved only with difficulty due to resistance from Serra da Estrela by Celts and Lusitanians led by Viriatus, who managed to resist Roman expansion for years. Viriatus, a shepherd from Serra da Estrela who was expert in guerrilla tactics, waged relentless war against the Romans, defeating several successive Roman generals, until he was assassinated in 140 BCE by traitors bought by the Romans. Viriatus has long been hailed as the first truly heroic figure in proto-Portuguese history. Nonetheless, he was responsible for raids into the more settled Romanized parts of Southern Portugal and Lusitania that involved the victimization of the inhabitants.

The conquest of the Iberian Peninsula was complete two centuries after the Roman arrival, when they defeated the remaining Cantabri, Astures and Gallaeci in the Cantabrian Wars in the time of Emperor Augustus (19 BCE). In 74 CE, Vespasian granted Latin Rights to most municipalities of Lusitania. In 212 CE, the Constitutio Antoniniana gave Roman citizenship to all free subjects of the empire and, at the end of the century, the emperor Diocletian founded the province of Gallaecia, which included modern-day northern Portugal, with its capital at Bracara Augusta (now Braga). As well as mining, the Romans also developed agriculture, on some of the best agricultural land in the empire. In what is now Alentejo, vines and cereals were cultivated, and fishing was intensively pursued in the coastal belt of the Algarve, Póvoa de Varzim, Matosinhos, Troia and the coast of Lisbon, for the manufacture of garum that was exported by Roman trade routes to the entire empire. Business transactions were facilitated by coinage and the construction of an extensive road network, bridges and aqueducts, such as Trajan's bridge in Aquae Flaviae (now Chaves).