The isopods of the family Cymothoidae are ectoparasites crustaceans that infest a wide variety of saltwater fish. This paper documented the first occurrence of Nerocila acuminata (Schiödte & Meinert, 1881) parasitizing the shortnose guitarfish Zapteryx brevirostris (Müller & Henle, 1841) (Rhinobatiformes, Rhinobatidae) in the southern Atlantic Ocean. This report can be an available tool to create a parasitology database for a species of conservation interest. It will also provide scientific data that can help the understanding of the biological factors acting in healthy populations and thus support the efforts made to preserve and conserve the species.