The marsh mongoose or water mongoose, (Atilax paludinosus), is a medium-sized mammal but a large mongoose. Weight can range from 2 to 5.5 kg (4.4 to 12 lb), with an average range of 2.5 to 4.1 kg (5.5 to 9.0 lb). From the head to the base of the tail, these animals range from 44 to 62 cm (17 to 24 in), with the tail adding 25–36 cm (9.8–14 in). It is a member of the mongoose family and the only member of its genus. Atilax paludinosus is found throughout sub-Saharan Africa, with a preference for permanent freshwater habitats bordered by dense vegetation, such as marshes, reed beds, and estuary (though sightings have been recorded in hilly areas with little or no aquatic wildlife presence). The marsh mongoose is an important member of the community of animals inhabiting the papyrus swamps, where deoxygenated water limits aquatic life to various air-breathing fish, frogs, insect larvae, snails, and mammals.

The marsh mongoose's dark brown fur (lighter on the undersides), with black-tipped guard hair, is long and coarse on the body and short about the hands and feet. The paws, unwebbed with underdeveloped pads, are soft and sensitive, resembling those of a raccoon, with the thumb passively enhancing grip on slippery surfaces. The claws are short and blunt and used for digging . There is little hair around the anus and on the upper lip. Typically, there are two pairs of mammae. The lower canines are well formed while the premolar teeth are thick and used for crushing hard foods. The thick tail narrows shortly to a point.