The woodchuck is the largest sciurid in its geographic range.
Males are slightly larger than females.
Woodchuck skulls are broad and flat. Strongly developed supraorbital processes project at right angles from the frontals with the tips projecting slightly forward. The dense, woolly underfur of the back and sides of the woodchuck is somewhat obscured by longer, less dense guard hair. The underfur of the woodchuck is gray with yellow tips, much paler from the dorsal area. Ears are small, low and rounded atop a black-colored head with small black eyes.