Heterosentis brasiliensis n. sp. (Acanthocephala, Arhythmacanthidae), parasitic in namorado sandperch Pseudopercis numida Miranda-Ribeiro, 1903 (Perciformes, Pinguipedidae) from the littoral of Cabo Frio, State of Rio de Janeiro, Brazil, is described and illustrated herein. Heterosentis brasiliensis n. sp. differs from all congeneric species by having 10 longitudinal rows of hooks in the proboscis, 6 or 7 hooks in each row, 3 or 4 small basal hooks in each row, and spines in the anterior ventral surface of the body. The similar species, Heterosentis heteracanthus (Linstow, 1896) and Heterosentis caballeroi Gupta & Fatma, 1983, also have 10 longitudinal rows of hooks, but H. heteracanthus differs from the new species by possessing trunk spines in the ventral and dorsal body surface. Heterosentis caballeroi differs from H. brasiliensis by the presence of 1 apical and 1 subapical hook in each longitudinal row; the largest apical, subapical, and basal hooks; lemnisci that are smaller than the proboscis receptacles; and a pre-equatorial male reproductive system. This is the first record of a Heterosentis species in a pinguipedid fish and from Brazilian coastal zone.