Cynoscionellan. g. is proposed for Cynoscionella sanmarcin. sp. and two previously described species of Diplectanum Diesing, 1858 were reassigned into the new genus. The three species are gill parasites of sciaenid fishes (Perciformes: Sciaenidae). The new genus is closely related with Diplectanum (sensu stricto), but is mainly distinguished by its male copulatory organ (MCO) having a slightly sclerotized sleeve-shaped base, lacking accessory piece and accessory copulatory organ. In addition, Cynoscionella n. g. differs from Diplectanum species by having the ventral anchors with well-developed roots, and a prostatic reservoir simple. Cynoscionella sanmarcin. sp. is a parasite of the cachema weakfish Cynoscion phoxocephalus Jordan & Gilbert, a demersal teleost collected from off the coastal zone of Puerto Pizarro, Tumbes region, northern Peru. Diplectanum bilobatum Hargis, 1955 from the spotted weakfish Cynoscion nebulosus (Cuvier) and D. monticellii Domingues & Boeger, 2003 from the smooth weakfish Cynoscion leiarchus (Cuvier), are transferred to Cynoscionellan. g. as Cy. bilobatum (Hargis, 1955) n. comb. and Cy. monticelli (Domingues & Boeger, 2003) n. comb. The present finding represents the first data on the monogenean parasites of C. phoxocephalus. This is the thirteenth marine diplectanid species recorded from Peru.