The average weight of the male Allen’s swamp monkey is around 7kg, substantially larger than the female at around 3kg. It is primarily a frugivorous species, but also consumes vertebrates, roots, and invertebrates.
The average group size is around 40 individuals. Groups will split up into subgroups of 2 to 6 individuals to forage for food. Sleeping sites for this species are usually located near water and the same sites are used repeatedly. Allen's swamp monkey is a semi-terrestrial and a diurnal species. This species will dive into the water when a predator is detected, and its slightly webbed feet help make it a good swimmer. It moves through the forest in a quadrupedal manner.