A new cichlid referable to the extant genus Gymnogeophagus is described from the Eocene Lumbrera Formation of the Salta Group, northwestern Argentina. †Gymnogeophagus eocenicus, n. sp., presents the synapomorphies that support the genus: absence of supraneurals and a presence of a forward spine in the first dorsal pterygiophore. The existence of an early to middle Eocene—aged species presenting the synapomorphies and the appearance of a modern genus requires the acceptance of an extensive differentiation from the basal cichlid lineages. Extant Gymnogeophagus species are restricted to the La Plata drainage and a few coastal drainages of southern Brazil and Uruguay. The occurrence of an Eocene fossil in the geographical area corresponding to the present distribution of the genus suggests the patterns of distribution and endemism of the Neotropical fish fauna have a very old history in the continent.