Leptarctus mummorum, new species, from the late Clarendonian of northwestern Kansas is described for the unusual mustelid genus Leptarctus Leidy, 1856. It is distinguished from other species of the genus by its larger size, temporal crests meeting along the central axis of the skull, relatively longer rostrum and bullar processes, depression anterior to the orbit, oblique ridge along the orbital wall, upper third premolar (P3) with an accessory internal accessory cusp, and upper fourth premolar (P4) with a greatly enlarged parastyle. It appears that previous suggestions that Leptarctus was strictly herbivorous and arboreal are not supportable based on the morphologies of the skull and dentition.