Objective was the relationship of the parasite Rhadinorhynchus plagioscionis with its host Plagioscion squamosissimus, providing morphological and morphometric data to prove the specificity of this parasite with its host, with reports occurring only in rivers in the Brazilian Amazon. Rhadinorhynchus plagioscionis infected P. squamosissimus captured in the municipal region of Santarém, in the state of Pará, in the eastern Brazilian Amazon. The specimens were analyzed by light and scanning electron microscopy. Morphological data obtained confirmed conspecificty with R. plagioscionis and supplemental information was added to the morphological concept of this species. The body of R. plagioscionis is cylindrical and milky-coloured with dorsal curvature, a long proboscis displaying ventral curvature with 12 longitudinal rows containing 22 to 24 hooks, which are larger and more prominent towards the end of the ventral curvature, and decrease in size in the posterior extremity of the proboscis. The present study describes, in addition to the morphological and morphometric characteristics of R. plagioscionis, which have host specificity for P. squamosissimus, a relationship being reported exclusively in the Brazilian Amazon.