A new species of Loimopapillosum Hargis, 1955 is described based on specimens collected from the gills of the diamond stingray, Hypanus dipterurus (Jordan & Gilbert, 1880) (Myliobatiformes: Dasyatidae) captured off Puerto Pizarro, Tumbes Region, northern Peru. Loimopapillosum pascuali n. sp. is distinguished mainly from the type and only species, Loimopapillosum dasyatis Hargis, 1955, by its funnel-shaped male copulatory organ, with an asymmetrical base; the superficial root of the anchor with distal knobs; the deep root of the anchor with a constriction at its base; a single testis; and the number of head organs. Available sequences for members of Monocotylidae in the GenBank as well as partial sequences for the gene 28S and 18S ribosomal DNA from L. pascuali n. sp. were included in phylogenetic analyses, suggesting that Loimoinae (represented in this study by L. pascuali n. sp. and Loimosina sp.) is nested within the Monocotylidae Taschenberg, 1879. Therefore, we confirm the rejection of Loimoidae Price, 1936 and its reincorporation as a subfamily of Monocotylidae, as previously suggested. Loimopapillosum pascuali n. sp. represents the first species of this genus in South America.