Hiodon is a Recent genus of fishes endemic to the fresh waters of North America; it is represented by two species: Hiodon tergisus Lesueur and H. alosoides (Rafinesque). Two specimens of osteoglossomorph collected from the Eocene-Oligocene Kishenehn Formation of northwestern Montana represent a new fossil species that is more closely related to the Recent Hiodon than it is to the Eocene †Eohiodon. The close relationship of this new species to Hiodon is supported by derived characters of the nasal, the opercular series, the jaws, the basihyal tooth plate, the infraorbitals, and the caudal skeleton. Comparative study of the postcranial skeleton of Recent and fossil hiodontids illustrates several stages in the evolution of the Hiodontidae: 1) origin of seven-rayed pelvic fins by the late Early Cretaceous (evidenced by †Yanbiania); 2) development of an anal fin larger than the dorsal fin and supported by 16 or more anal pterygiophores at least as early as the early Late Cretaceous (evidenced by †Plesiolycoptera); 3) evolution of a falcate dorsal fin and a sexually dimorphic anal fin at least as early as the Eocene (evidenced by †Eohiodon); and 4) increase in number of vertebrae accompanied by changes in the caudal skeleton as seen in the new fossil species and in Recent species of Hiodon.