Gnathorhiza is an extinct genus of prehistoric lobe-finned fish (lungfish) which lived from the Carboniferous period to the Early Triassic epoch.

Cope stated in the original description of the species that it may belong to the petalodont family, though he personally found that doubtful and thought the tooth roots were more like those of sharks.

In 1934, Romer and Smith assigned the genus to the family Lepidosirenidae on the basis that Gnathorhiza exhibited a similar shearing motion of the jaw to extant Lepidosiren and Protopterus.

[4] In 1977, Gnathorhiza would be moved to a new family, Gnathorhizidae, which was thought to be the sister group to the extant Lepidosirenidae and Protopteridae based on morphological evidence.

The geologically youngest record of the species is from the uppermost Olenekian of Russia, where it is outnumbered by other lungfish genera (including Arganodus, Ceratodus and Ptychoceratodus) which may have replaced it.

[1] Because specimens have been found in Carboniferous and Permian strata representing freshwater, estuarine and marine environments, it is believed that Gnathorhiza was a euryhaline lungfish, capable of adapting to a wide range of salinities.