Introduction A new monogenean, Heterocotyle margaritae n. sp. (Monocotylidae: Heterocotylinae), is described based on specimens collected from the gills of the diamond stingray, Hypanus dipterurus (Jordan & Gilbert, 1880) (Myliobatiformes: Dasyatidae) captured of the South Pacifc coast, Lima Region, Peru. Materials and methods Monogeneans were fxed in hot 4% formalin, observed and measured as permanent mounts stained with Gomori’s trichrome, and mounted in Canada balsam. Drawings were made with the aid of a drawing tube. Results The new species is distinguished from all congeners mainly by having a funnel-shaped male copulatory organ, spatulate distally with lateral folds and by its club-shaped accessory piece. The most similar congener, H. americana Hargis, 1955, resemble H. margaritae n. sp. regarding the general structure of the male copulatory organ (funnel-shaped with an accessory piece). However, besides other features, the new species difer from H. americana by having an accessory piece without a dorsal button (vs an accessory piece with a dorsal button in H. americana). Conclusions This is the frst report of a member of Heterocotyle Scott, 1904 in the Southeastern Pacifc Ocean. The present fnding brings to 20 the number of known species of Heterocotyle, and represents the ffth described monogenean species from Hypanus dipterurus.