Belgæ, the name given by Cæsar to the war-like tribes which in his time occupied that one of the great divisions of Gallia bounded on the N. by the Rhine, on the W. by the ocean, on the S. by the Sequana (Seine) and Matrona (Marne), and on the E. by the territory of the Treviri. Their country was level, containing no mountains of any height, except the Vosges in the south. The name seems to have originally designated several powerful tribes inhabiting the basin of the Seine, and to have been afterwards used by Cæsar as a general appellation for all the peoples north of that river. Most probably they were chiefly of Celtic origin, but within their territories were to be found both pure and mixed Germans.
When South Britain was invaded by Cæsar, he found that Belgæ from the opposite shores of Gaul had preceded him, and were settled in Kent and Sussex, having driven the aborigines into the interior. The Belgæ in Britain resisted for nearly a century the Roman power, but were finally forced to yield to it. Cæsar regarded them as German, but they rather seem to have belonged to the Celtic portion of the Gallic Belgæ. Certainly none of the names of their three towns are Germanic. See Rhys's Celtic Britain (1882), and Elton's Origins of English History (1882).