The Segni (sometimes Segui) were a tribe living in Belgic Gaul when Julius Caesar's Roman forces entered the area in 57 BCE. They are known from his account of the Gallic War. They were one of a group of tribes listed by his local informants as the Germani of Belgian Gaul, along with the Eburones, Condrusi, Paemani (or Caemani), and Caeroesi (or Caeraesi). The Segni do not appear in the first listing of the Germani, which was a listing of Germani sending men to fight Caesar. But they appear in a later mention, after the defeat of the Eburones:
The Segui and Condrusi, of the nation and number of the Germans [Germani], and who are between the Eburones and the Treviri, sent embassadors to Caesar to entreat that he would not regard them in the number of his enemies, nor consider that the cause of all the Germans on this side the Rhine was one and the same; that they had formed no plans of war, and had sent no auxiliaries to Ambiorix. Caesar, having ascertained this fact by an examination of his prisoners, commanded that if any of the Eburones in their flight had repaired to them, they should be sent back to him; he assures them that if they did that, he will not injure their territories.
These tribes are referred to as the "Germani Cisrhenani", to distinguish them from Germani living on the east of the Rhine, outside of the Gaulish and Roman area. Whether they actually spoke a Germanic language or not, is still uncertain. The region was strongly influenced by Gaul, and many of the personal names and tribal names from these communities appear to be Celtic. But on the other hand it was claimed by Tacitus that these Germani were the original Germani, and that the term Germani as it came to be widely used was not the original meaning. He also said that the descendants of the original Germani in his time were the Tungri.
The general area of the Belgian Germani was between the Dijle (Dyle) and Rhine rivers, and north of Luxemburg and the southern parts of the Eifel. In modern terms this area includes eastern Belgium, the southeastern parts of the Netherlands, and a part of Germany on the west of the Rhine, but north of the Moselle valley.