Theromyzon Philippi, 1867 is a genus of sanguivorous, freshwater leeches in the family Glossiphoniidae. The genus is broadly distributed across the globe, possibly due to the frequent feeding in the nasopharyngeal cavities of migratory waterfowl that may allow for long distance dispersal. The genus has a history of taxonomic confusion resulting from mischaracterisations of key morphological features of type specimens that have produced several re-descriptions and synonyms. Here, we bring partial order to this confusion through robust morphological investigations of newly collected North American (and a single South American) specimens, representing most of the known species diversity from this continent. We also produce the first species-level phylogeny for Theromyzon and attempt to understand species boundaries regarding both morphology and genetics. Our results demonstrate that there are at least five species of Theromyzon present in North America (T. bifarium, T. tessulatum, T. rude, T. trizonare, and a clade that needs further investigation), and a hitherto undescribed taxon that does not conform to any previously published description, and represents a unique lineage in the phylogeny; we describe this new species under the name Theromyzon tigris sp. nov. This study sheds light on the discriminatory power of select morphological characters and the distribution of phenotypes within the genus. We also provide a comprehensive classification framework for the known species within the genus designed to facilitate identification and minimise future taxonomic confusion.