Hisonotus insperatus, new species, is distinguished from all the other species of the genus Hisonotus Eigenmann and Eigenmann by the combination of a pair of rostral plates at the tip of the snout, the complete median plate series to base of the caudal fin, a lateral line that is continuous along the middle of the body, a pectoral spine smooth and lacking odontodes along its posterior dorsal margin for a distance more than one-half of its width, and a scapular bridge that is completely exposed, sometimes with a very reduced median pair of arrector fossae.