The proteocephalidean cestode Harriscolex nathaliae n. sp. (Proteocephalidae: Zygobothriinae) is described from the intestine of the spotted sorubim Pseudoplatystoma corruscans (Spix and Agassiz) (Siluriformes: Pimelodidae) from the Paraná River basin in Argentina. This new species differs from the only species of the genus, Harriscolex kaparari (Woodland, 1935), which is a parasite of Pseudoplatystoma tigrinum (Linnaeus) from the Amazon River in Brazil, by its larger scolex (width of 450–750 μm vs. 305–340 μm), the position of the vagina in relation to the cirrus sac (anterior and posterior vs. only anterior), an asymmetrical vaginal sphincter, the arrangement of vitelline follicles (1 narrow longitudinal band on each side of the proglottid vs. 2 pairs of wide longitudinal bands on dorsal and ventral sides), and a higher number of uterine diverticula (22–45 vs. 16–20). Harriscolex nathaliae is covered with 2 types of microtriches, acicular filitriches and gladiate spinitriches.