The Belgae were a group of tribes living in northern Gaul, on the west bank of the Rhine, in the 3rd century BC, and later also in Britain, and possibly even Ireland. They gave their name to the Roman province of Gallia Belgica, and very much later, to the modern country of Belgium.

The general consensus among linguists is that the ethnic name Belgae comes from the Proto-Celtic root *belg- or *bolg- meaning "to swell (particularly with anger/battle fury/etc.)", cognate with the Old English verb belgan, "to be angry" (from Proto-Germanic *balgiz), derived ultimately from the Proto-Indo-European root *bhelgh- "to swell, bulge, billow"). Thus, a Proto-Celtic eethnic name *Bolgī could be interpreted as "The People who Swell (particularly with anger/battle fury)".

Julius Caesar describes Gaul at the time of his conquests (58 - 51 BC) as divided into three parts, inhabited by the Aquitani in the southwest, the Gauls of the biggest central part, who in their own language were called Celtae, and the Belgae in the north. Each of these three parts were different in terms of customs, laws and language. He noted that the Belgae, being farthest from the developed civilization of Rome and closest to Germania over the Rhine, were the bravest of the three groups, because "merchants least frequently resort to them, and import those things which tend to effeminate the mind".