The proteocephalid genus Pseudoendorchis (Cestoda: Onchoproteocephalidea) has recently been proposed to accommodate seven species/species-level lineages of tapeworm parasites of catfishes (Siluriformes) in the Neotropical region, based on integration of genetic information, and morphological and ecological data. Its members are typified by having a large Mehlis' gland, representing more than 1/5 (usually 1/4-1/2) of proglottid width, and the vagina always anterior to the cirrus-sac. Critical examination of previously unstudied museum cestodes tentatively designated as Pseudoendorchis sp. 3 from Megalonema platycephalum (Pimelodidae) in the Peruvian Amazon made it possible to formally describe this taxon and to differentiate it from all congeneric species. The new species, which is the first parasite of M. platycephalum ever recorded, is characterised mainly by having the scolex bearing four uniloculate suckers (biloculate in all nominal species of the genus), the lowest relative surface of the ovary (ratio of its surface to that of the whole proglottid) among species of the genus ( 11%, usually 15-20%), and an extraordinarily large Mehlis' gland (its diameter represents 41-50% of proglottid width). Morphology of the terminal portion of the vaginal canals in proteocephalids is briefly discussed.