The first recorded mention of Belgae, part of the mix that make up modern Belgians, was in the year 58 B.C.; Gaius Julius Caesar, departing from the Roman province of Gallia Narbonensis (now Provence), decided to conquer the rest of the Gauls. In the northern part of Gaul he had to fight in 56 B.C. a series of tribes, which Strabo said numbered fifteen in all. The Belgae occupied lands on the left bank of the Rhine,

Whether the Belgae were Celts or Germans occupied 19th century and early 20th century historians. In his book De Bello Gallico ("The Gallic War") Caesar records that Gallia is divided into three parts, inhabited by the Aquitani, Belgae and the Gauls. This would imply that the Belgae were different from the Gauls. On the other hand, he writes about the Gauls: horum omnium fortissimi sunt Belgae (out of all of those Gauls the Belgians are the most brave). Other tribes that Caesar may have been including among the Belgae were the Leuci, Treveri and Mediomatrici. Poseidonius includes the Armoricani in Brittany as well.

After the conquest their homeland has been indicated by Roman authors as "Gallia Belgica". The region reached almost from Paris in the south up to the river Rhine in the north and the east.

The Belgae had made their way across the English Channel into southern England in Caesar's time (Bello Gallica ii:4 and v:12), and settled in some of the southern counties where among their towns were Magnus Portus (Portsmouth) and Venta Belgarum (Winchester).

Caesar attacked the Belgae in 57 BC and defeated the Nervii, Atrebates and their allies in a close pitched battle, the Nervii were completely decimated. That same year Caesar attacked and defeated the Aduatuci who were marching to the aid of the Nervians but didn't arrive in time for the battle. In 53 BC Ambiorix of the Eburones wiped out 15 cohorts and the Eburones, Nervii, Menapii and Morinii revolted again only to be put down by Caesar. The Belgae fought in the uprising of Vercingetorix, 52 B.C..

After their final subjugation, Caesar combined the three parts of Gaul, the territory of the Belgae, Celtae and Aquitani into a single unwieldy province (Gallia Comata) that was reorganized by Augustus into its traditional cultural divisions. Roman Belgica was bounded on its east by the Rhine and extended all the way from the North Sea to Lake Constance (Lacus Brigantinus), including parts of what is now western Switzerland, with its capital at the city of the Remi (Reims).

Under Diocletian, Belgica Prima (capital, Augusta Trevirorum, Trier) and Belgica Secunda (capital Reims) formed part of the diocese of Gaul.