Irregular dark blotches on dorsum and sides. No external gills. Gill slits present, but difficult to detect. Front feet with four toes and hind feet with five toes. Similar species include mudpuppies and waterdogs, but both have external gills and four toes on the hind feet.
Adults lack eyelids and retain one pair of gill slits.
Hellbenders also express unusual specializations associated with cutaneous respiration. Fleshy folds along the body and legs increase the surface area of the skin available for gas exchange. The folds are highly vascularized with capillaries running close to the surface. In moving water, the current carries away CO2 and brings oxygenated water into contact with the skin. In still water, the hellbender renews the surrounding layer of water by gently swaying back and forth.