Cottus caeruleomentum (Blue Ridge Sculpin) is described from the Elk, Susquehanna, Bush, Patapsco, Patuxent, Potomac, Nanticoke, James, and Roanoke drainages of the middle Atlantic eastern United States. The new species is a member of the bairdi group with the following characters: sexual dimorphism in breeding features, two-spotted first dorsal fin, chin uniformly pigmented, dorsal saddles moderate to wide, incomplete lateral line, preopercular spines slight to moderate, palatine teeth weak to moderate, and postpectoral prickling slight to moderate. Cottus caeruleomentum is differentiated from C. b. bairdi by (the first character state refers to C. caeruleomentum): (1) caudal base band unnotched in at least one side versus both sides notched; (2) pectoral fin rays modally 14 versus modally 15; and (3) electrophoretic mobility at the Ck-A locus fast versus slow. Cottus caeruleomentum is a variable species with interdrainage differences in morphology and allozymes. Parsimony analysis of allozyme data and one morphological character using Cottus girardi as the outgroup revealed C. caeruleomentum is monophyletic with respect to the three populations of C. b. bairdi included in this study. Data suggest C. caeruleomentum dispersed through the extended greater Susquehanna River during the Pleistocene epoch.