Allen's swamp monkeys are small and stout. Their fur is brown, gray and green and grows longer around the neck and shoulders than on the rest of the body. Their tails are generally longer than their bodies.

Adults may reach 18 inches (45.7 centimeters) in body length with tails over 19 inches (48.2 centimeters) in length. Males are noticeably larger than females and can exceed 13 pounds (5.9 kilograms), while females are closer to 7.5 pounds (3.4 kilograms).

Allen's swamp monkeys are native to central Africa and are primarily concentrated in the lowland forests of the Congo basin, which includes Cameroon, Congo and Democratic Republic of the Congo. As their name suggests, Allen's swamp monkeys inhabit swamp forests. Water is central to their habitat, and groups may even choose to sleep near bodies of water.

The life span of the Allen's swamp monkey is estimated to be about 28 years.

To communicate with members of their troop, Allen's swamp monkeys use gestures and calls. Their vocalizations include warning chirps, deep croaks and grunts.

Allen's swamp monkeys are omnivores and eat a variety of fruits, seeds, insects, fish, shrimp, snails, small invertebrates and leaves. They forage on the ground and in shallow water.

At the Smithsonian's National Zoo, Allen's swamp monkeys receive a diet consisting of a complete primate biscuit supplemented with vegetables, greens and fruits, and a small amount of root vegetables.

Allen's swamp monkeys are diurnal, meaning they are active during the day.

A highly social primate, the swamp monkey lives in groups of 40 or more individuals. Like many other primate species, the swamp monkey practices social grooming, strengthening troop bonds by grooming each other. Smaller groups break off to forage. Though they are largely arboreal, spending much of their time in the trees, the semi-terrestrial monkeys will come to the ground to forage.

Allen's swamp monkeys live in social groups with multiple males and females. Females give birth to a single young or, very rarely, twins after a five to six month gestation period. The females are thought to be the primary care givers. Young are typically weaned after about three months and are extremely active. Births of Allen's swamp monkeys have been recorded throughout the year.

The Allen's swamp monkey is considered to be a species of least concern, though it is protected by the Convention on the International Trade in Endangered Species and the African Convention on the Conservation of Nature and Natural Resources.

The primary threats to the monkeys include hunting for bushmeat, killing in retaliation for crop raids and collection for the pet trade. IUCN states a need for further studies on the impact of hunting on the population.