In 188 BCE, the Romans invaded Thrace, and warfare continued until 46 CE when Rome finally conquered the region. The Odrysian kingdom of Thrace became a Roman client kingdom c. 20 BCE, while the Greek city-states on the Black Sea coast came under Roman control, first as civitates foederatae ("allied" cities with internal autonomy). After the death of the Thracian king Rhoemetalces III in 46 CE and an unsuccessful anti-Roman revolt, the kingdom was annexed as the Roman province of Thracia. The northern Thracians (Getae-Dacians) formed a unified kingdom of Dacia, before being conquered by the Romans in 106 and their land turned into the Roman province of Dacia.

In 46 CE, the Romans established the province of Thracia. By the 4th century, the Thracians had a composite indigenous identity, as Christian "Romans" who preserved some of their ancient pagan rituals. Thraco-Romans became a dominant group in the region, and eventually yielded several military commanders and emperors such as Galerius and Constantine I the Great. Urban centres became well-developed, especially the territories of Serdika, what is today Sofia, due to the abundance of mineral springs. The influx of immigrants from around the empire enriched the local cultural landscape. Sometime before 300 CE, Diocletian further divided Thracia into four smaller provinces.