Placobdella parasitica (Say, 1824) is one of the most commonly encountered turtle leeches in North America. Molecular analysis of individuals of P. parasitica representing various populations throughout its range in North America using the cytochrome c oxidase subunit I gene, revealed the presence of nine distinct groups: (1) P. parasitica sensu stricto containing members, including the neotype specimen, occurring broadly throughout the central United States westward from the Mississippi River to the Rocky Mountains and throughout southern Ontario, Canada, and the upper midwestern United States as far east as New York State; (2) West Virginia; (3) Mississippi/Alabama; (4) Northeast, including New York, Massachusetts, and Vermont; (5) New England, including Rhode Island, Massachusetts, and Connecticut; (6) North Carolina/West Virginia; (7) South Carolina; (8) Tennessee; and (9) Florida. Both neighbor-joining and maximum likelihood analyses recovered an east-west split along the Appalachian Mountains with groups 1–3 clustering together and groups 4–9 clustering separately, with the exception of group 8 (Tennessee) that placed with the eastern groups. Group 1 includes specimens from a broad geographic distribution, yet with relatively low genetic variation, a pattern observed in other glossiphoniid species in North America. The groups with members east of the Appalachian Mountains are more tightly clustered by locality. This leech species is known to parasitize several turtle species, including Chrysemys picta, the painted turtle that originated in the central Gulf Coast region and dispersed northward representing a recolonization after Pleistocene glaciation. The neighbor-joining tree and pairwise distance data could suggest that P. parasitica has a similar phylogeographic pattern and dispersal history with its turtle hosts. In view of the morphological uniformity among the various groups, P. parasitica is provisionally considered to be a widely distributed, molecularly variable species.