Potamotrygonocestus Brooks & Thorson, 1976 is currently represented by six recognised species of tetraphyllidean cestodes inhabiting Neotropical freshwater stingrays. Potamotrygonid stingrays examined to date have included only a single specimen of Plesiotrygon iwamae. Only one species of tetraphyllidean, Potamotrygonocestus chaoi Marques, Brooks & Araujo, 2003, has been described from this host, and this description was based on limited material. New efforts to document the diversity in this host species resulted in the collection of eight additional specimens of P. iwamae, one of these from the upper Rio Solimões, at São Paulo de Olivença Amazonas during September, 2003, and seven from the Baía do Marajó, Pará, during November, 2003. The specimen from the upper Solimões was found to be infected with P. chaoi. Voucher material from this stingray was used for the redescription of this cestode, which is characterized by strobila 8.78-22.83 mm long and a great number of proglottides, 58-93; the new material provided strobilar length and proglottis counts for complete worms. Potamotrygonocestus marajoara n. sp. is the second species of this genus reported from Plesiotrygon iwamae, although it appears to be restricted to the lower Amazon. This new species resembles P. chaoi in possessing filitriches and blade-like spinitriches on the scolex, cephalic peduncle and cirrus, but differs from other species of the genus in the number of testes, which is 44 on average per proglottis, and by having apical sucker measuring 95-175 microm in length. Additional data on the distribution and morphology of the microtriches and a detailed description of the female reproductive system are also provided in this study.