Firmus (died 375) was a Berber Numidian prince and Roman usurper under Valentinian I. Firmus was the son of the Berber Jubaleni prince Nubel, a powerful Roman military officer, as well as a wealthy Christian. When Nubel died, Firmus killed his half-brother Zammac, who had illegitimately appropriated Nubel's wealth, and became successor to his father. Between 372 and 375, Firmus revolted against the comes Africae Romanus, who was a supporter of Zammac.