From the newly conquered land. today's Romania and a part of Bulgaria, Moldavia and Hungary, Trajan organized the Roman province of Dacia, whose capital, Ulpia Trajana, stood on the site of Sarmizegetusa. Many Getae resisted Roman authority and some fled northward, away from the centers of Roman rule. Trajan countered local insurrection and foreign threat by stationing two legions and a number of auxiliary troops in Dacia and by colonizing the province with legionnaires, peasants, merchants, artisans, and officials from lands as far off as Gaul, Spain, and Syria. Agriculture and commerce flourished, and the Romans built cities, fortresses, and roads that stretched eastward into Scythia.

In the next 200 years, a Dacian ethnic group arose as Roman colonists commingled with the Getae and the coastal Greeks. Literacy spread, and Getae who enlisted in the Roman army learned Latin. Gradually a Vulgar Latin tongue superseded the Thracian language in commerce and administration and became the foundation of modern Romanian. A religious fusion also occurred. Even before the Roman invasion, some Getae worshiped Mithras, the ancient Persian god of light popular in the Roman legions. As Roman colonization progressed, worshipers faithful to Jupiter, Diana, Venus, and other gods and goddesses of the Roman pantheon multiplied. The Dacians, however, retained the Getian custom of cremation, though now, amid the ashes they sometimes left a coin for Charon, the mythological ferryman of the dead.

The conquest of Dacia by the Romans and its turning into an imperial province (A.D. 106-271) brought about major changes in the native population's economic, social and political life. The Geto-Dacians continued to remain the main ethnic community both in the free and in the occupied territories. They continued to work side by side with the Roman colonists and veterans, who had been brought into the new Imperial province of Dacia from everywhere in the Roman World. The spirit of the conquerors, backed by the diligence of the local population, proved very profitable for the country: Dacia reached a high level of material and spiritual culture. The intense process of Romanization stamped a lasting mark on the language of the Romanian people, on their name, conscience and culture. The Romanian people's formation relied on two basic ethnic elements, namely the Geto-Dacians, and the Romans, who superposed, with a minor Slavic adjustment.

The crisis that shook the Roman Empire in the 3rd century, as well as the pressure exerted by the "Barbarian" populations, made Emperor Aurelianus (A.D. 270-275) withdraw his troops, administrative body and part of the urban population from Dacia southward, across the Danube (A.D. 271), where Dacia Aureliana was set up. However, most of the population, made up of Roman colonists and Romanized Dacians, stayed on and continued to keep up close relations with the South-Danubian Romans. These relationships were very close indeed, as attested by rich archaeological findings in Transylvania (Alba-Iulia, Bratei), Oltenia, Wallachia (Sucidava, Romula, Câmpulung-Muscel), and even in Moldavia, as well as by the wealth of coin hoards which can be found everywhere on present Romania's territory.

The process of Romanization went on north of the Danube after the 3rd century as well. This was largely due to the Christian faith which was spreading out from towns situated on the right bank of the middle and lower streams of the Danube. Some Roman emperors, and subsequently some of the Byzantine ones, would raid the north-Danubian areas, managing, under Constantine the Great (307 - 337), Valens (364 - 378) and Justinian (527 - 565), to partially restore Roman rule over the former Dacia province.

The "Barbarian" waves that swept across Dacia's territory, i.e. Goths, Huns, Avars, Slavs changed its social and political organisation. Like in other parts of Europe, the barbarians largely destroyed town networks, and, consequently, the core of economic activities shifted from cities to the countryside, which brought about a process of ruralization of the entire society. The Daco-Roman population gathered together in what the Romanian historian Nicolae Iorga would call popular Romanii. The inhabitants of these territories developed a sense of their belonging, or of their having belonged to the Roman Empire. Their main occupation was the cultivation of land and the breeding of animals; their Roman ancestry is still reflected in the Romanian language, as the names of the chief occupations and farm products in Romanian are of Latin origin.