Priacanthids are a small family of percomorph fishes comprising fewer than 20 extant species currently assigned to four genera. One of these, Pristigenys, was established by Louis Agassiz (1835) to include the Eocene species Pristigenys substriata from Monte Bolca, and is usually regarded as a subjective senior synonym of Pseudopriacanthus. Consequently, Pristigenys currently comprises five extant species plus the fossil Pristigenys substriata. The osteology of the type species of this genus, however, is poorly known, and this makes it difficult to provide an adequate comprehensive definition of the taxonomy of the whole family. Pristigenys substriata is redescribed in detail based on five well-preserved articulated skeletons. Pristigenys substriata can be easily distinguished from other priacanthids based on its unique combination of characters. Morphological analysis of the fossil specimens reveals that there is substantial evidence to justify recognition of both Pristigenys and Pseudopriacanthus as valid genera, with extant species previously assigned to Pristigenys now referred to Pseudopriacanthus. Within the Priacanthidae, Pristigenys and Pseudopriacanthus form sister taxa and this pair can be considered as the sister-group to all remaining extant priacanthid genera (Cookeolus [Heteropriacanthus Priacanthus]).