Lungfishes (dipnoans) are lobe-finned fishes that have lived from the Devonian to Recent. Here, we describe a previously unreported Mesozoic lungfish species assigned to the genus Ceratodus on the basis of an isolated upper tooth from the Western Interior of North America. The morphology of the tooth specimen clearly suggests that it represents an undescribed taxon, but although distinct, the species cannot be given a new taxonomic name because of its mysterious provenance data. The specimen is noteworthy because it is the largest known lungfish tooth reported to date. We estimate the total length of this durophagous fish to be about 4 m in life. This discovery adds a remarkable new component to the paleoecology of North America during the ‘Age of Reptiles.’