Marsh rabbits are medium-sized rabbits with short, rounded ears and small feet. The head and tail is smaller than those of easten cottontail rabbits (Sylvilagus floridanus). Marsh rabbits are dark brown to reddish brown with a dark belly. The characteristic that distinguishes S. palustris from all other cottontail rabbits is the dark color of the underside of their tail. Other cottontail rabbits have white undersides to their tails. Marsh rabbits have sparse fur and remarkably long toenails on their hindfeet. There is no substantial sexual dimorphism.

Range mass: 1,200 to 2,200 g.

Range length: 425 to 440 mm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: sexes alike