The Proboscis Monkey (Nasalis larvatus, monotypic in genus Nasalis) is a reddish-brown arboreal leaf-eating monkey, found only in the coastal areas of Borneo and the Mentawai Islands west of Sumatra, in coastal mangrove swamps and riverine forests.

The most distinctive trait of these monkeys is the males' large protruding nose. The purpose of the large nose is unclear, but it has been suggested that it is simply a result of sexual selection, i.e., female proboscis monkeys prefer big-nosed mates, thus propagating the trait.

Males are much larger than females. Males can reach 72 cm (28 inches) in length, with an up to 75 cm tail, and weigh up to 24 kg (53 pounds). Females are up to 60 cm long, weighing up to 12 kg (26 pounds).

Proboscis monkeys also have large bellies, as a result of their diet. Their digestive system is divided into several parts, with distinctive gut flora, which help them digest leaves. This digestive process releases a lot of gas, resulting in the monkeys' "bloated" bellies. A side-effect of this unique digestive system is that they are unable to digest sweet fruit , unlike most other simians. They mostly eat green fruits, seeds, and leaves.

They live in small groups of 10 to 32 animals. Group membership is very flexible, and animals are known to move from group to group quite often.

The proboscis monkey habitat is both arboreal and amphibious, with their mangrove swamp and riverine environment containing forest, dry land, shallow water allowing wading, and deep water requiring swimming. Like other monkeys, proboscis monkeys climb well. They are also proficient swimmers, often swimming from island to island, and have been picked up by fishing boats in open ocean a mile from shore. While wading, the monkeys use an upright posture, with the females carrying infants on their hip. Troops have been filmed continuing to walk upright, in single file, along forest trails when they emerge on land, the only non-human mammal known to use this form of locomotion for any length of time.

These monkeys are highly endangered, and only about 7000 are known to still exist in the wild.