Ameloblastella martinae n. sp. is described from the gills of the pimelodid catfishes (Siluriformes: Pimelodidae) Sorubim lima (Bloch & Schneider) (type-host) and Hemisorubim platyrhynchos (Valenciennes) in the Peruvian Amazonia, and on Pseudoplatystoma corruscans (Spix & Agassiz) and P. recticulatum Eigenmann & Eigenmann in Argentina. The new species is distinguished from other congeners mainly by the morphology of the male copulatory organ (MCO), which has a form of a corkscrew with tight rings, whereas in other species of Ameloblastella Kritsky, Mendoza-Franco & Scholz, 2000, the MCO is formed by a delicate and coiled tube forming loose rings. Sclerotised structures (haptoral elements and MCO) of specimens of A. martinae n. sp. were used to compare two parasite populations (from Peru and Argentina) using Euclidean distances. Despite the geographical isolation and different host-associations, both populations belong to the same species. The phylogenetic position of A. martinae n. sp. was analysed using partial sequences of the 28S rDNA gene along with 46 species of dactylogyrid parasites of siluriforms (Siluriformes) under Maximum Likelihood (ML) and Bayesian Inference (BI) criteria. Phylogenetic reconstructions showed that Ameloblastella represented by five species, including its type-species A. chavarriai (Price, 1936) from the heptapterid Rhamdia guatemalensis and A. martinae n. sp., was recovered as a well-supported monophyletic group (in both analyses, ML and BI). An additional species, Ameloblastella sp., was found on P. corruscans and P. reticulatum in Argentina. The morphology of the MCO and haptoral elements suggests that Ameloblastella sp. may represent a new species. However, the few specimens found and the lack of genetic sequences of this species precluded its formal description.