Understanding the status and distribution of species is fundamental for conservation. However, recent genetic work has challenged the known distributions of some unionid taxa. The recognized range of the Louisiana Fatmucket Lampsilis hydiana spans watersheds from east Texas northward to southern Arkansas and eastward to western Mississippi. Specimens with morphological similarities to L. hydiana have been observed in Illinois and were presumed to be Fatmucket Lampsilis siliquoidea based on known distributions of Lampsilis species in Illinois. We examined specimens from Illinois and completed comparative genetic analyses using the mitochondrial genes cox1 and nad1 for species resembling L. siliquoidea. Our results show two morphologically similar, yet genetically distinct, species in Illinois. One of these species was genetically similar to L. siliquoidea, and one of these species showed little-to-no genetic difference from topotypic L. hydiana. The confirmation of L. hydiana populations within Illinois is significant for documenting the faunal diversity of the state. The varying degree of phenotypic separation confirms the need for further morphological research within Lampsilis, as well as genetic research throughout the updated known range of L. hydiana.