Gyrodactylus mediotorus n. sp. is described from the body, fins, and buccal cavity of the spottail shiner, Notropis hudsonius (Cyprinidae) from the St. Lawrence River, Quebec, Canada. Gyrodactylus mediotorus n. sp. is the first species of Gyrodactylus Nordmann, 1832 described from N. hudsonius and is characterized by large hamuli, large medial process of the ventral bar, narrow linguiform ventral bar membrane, large anterolateral processes, and marginal hooks with long shafts and distinctly shaped sickle. The species that most resembles Gyrodactylus mediotorus n. sp. is Gyrodactylus protuberus Rogers and Wellborn, 1965 described from the stargazing shiner, Notropis uranoscopus Suttkus, 1959. The 2 species can be differentiated based on the larger hamuli (68.4 vs. 64) and ventral bar (38.4 vs. 24) of Gyrodactylus mediotorus n. sp. and the shape of the marginal hooks which for Gyrodactylus mediotorus n. sp. has a slightly larger toe and a point which is not as angled. The morphological description is supplemented with 436 sequenced base pairs of the 18S gene (including the V4 region) as well as 1,041 sequenced base pairs spanning the complete ITS-1, 5.8S, and ITS-2 regions. BLAST (Basic Local Alignment Search Tool) searches failed to provide any close matches for either regions of DNA, with Gyrodactylus colemanensis infecting Salvelinus fontinalis being the most genetically similar for both the 18S (∼91%, JF836090) and ITS (∼84%, JF836142) rDNA regions. Gyrodactylus mediotorus n. sp. has been found infecting spottail shiners in the St. Lawrence River in low prevalence and intensities periodically over the last 15 yr.