Individuals of the ground beetle genus Diplous Motschulsky, 1850 occur in riparian areas predominately throughout boreal North America and Asia. In order to infer the species phylogeny of the North American Diplous, we examined 97 morphological characters (56 quantitative characters and 41 qualitative characters) and 458 bp of the mitochondrial cytochrome oxidase subunit I region. We used the four North American species, four Palearctic species, and one undescribed species of a closely related genus to test the monophyly and the direction of character state change in North American Diplous. Overall, we found that North American Diplous appear to represent a monophyletic group, but that the morphological and molecular evidence did not support the same relationships in the placement of one of the species. We found that the total evidence trees agreed most with biogeography and considerations of accelerated morphological evolution. In this paper, we present a morphological phylogenetic tree, a molecular phylogenetic tree, a total evidence phylogenetic tree, a species key, species diagnoses, and a distribution map of Nearctic Diplous.