Kelemejtubus castroi, gen. et sp. nov., is described based on six specimens from the marine Danian deposits exposed in the Belisario Domínguez and División del Norte quarries, both located near Palenque City, Chiapas, México. Kelemejtubus castroi shows sufficient features to support its inclusion into the series Percomorpha and is diagnosed by the unique combination of an elongated maxillary, reaching the posterior portion of the orbit; the premaxillary and dentary bearing a row of stout conical teeth; an anterior longer canine-like tooth in the lower jaw; no supramaxillary; preopercle with three antrorse spines on its ventral portion; opercle with two posterior spines; absence of dorsal and anal fin stays; two slightly separated dorsal fin lobes, the first with nine spines and the last with one spine plus nine rays; an anal fin composed of two anal spines and seven soft rays; one supernumerary spine on the first anal pterygiophore; 24 vertebrae, with 10 being abdominal; a caudal fin formula I 8-7 I; three epurals; and a slightly forked posterior ventral procurrent ray with a spur. The absence of unique derived characters, however, does not allow its assignment to any subgroup; thus, it is considered here as a percomorph incertae sedis. This taxon increases the knowledge about the fish assemblage that lived in marine environments at the extreme south of North America approximately 63 million years ago, just after one of the most catastrophic massive extinctions responsible for drastic changes in biodiversity.