The proboscis monkey or long-nosed monkey (Nasalis larvatus) is an arboreal Old World monkey with an unusually large nose (or proboscis), a reddish-brown skin color and a long tail. It is endemic to the Southeast Asian island of Borneo and is found mostly in mangrove forests and on the coastal areas of the island.

This species co-exists with the Bornean orangutan and monkeys such as the silvery lutung. It belongs in the monotypic genus Nasalis.

The proboscis monkey belongs to the subfamily Colobinae of the Old World monkeys. The two subspecies are:

However, the difference between the subspecies is small, and not all authorities recognise N. l. orientalis.

The genus name Nasalis comes from the Latin word nasus meaning "nose".^{[further explanation needed][additional citation(s) needed]}

This animal was made known to Westerners by Baron Friedrich von Wurmb in 1781, he later sent specimens of it to Stamford Raffles in Europe.^{[additional citation(s) needed]}

The proboscis monkey is a large species, being one of the largest monkey species native to Asia. Only the Tibetan macaque and a few of the gray langurs can rival its size.

Sexual dimorphism is pronounced in the species. Males have a head-body length of 66 to 76.2 cm (26.0 to 30.0 in) and typically weigh 16 to 22.5 kg (35 to 50 lb), with a maximum known weight of 30 kg (66 lb). Females measure 53.3 to 62 cm (21.0 to 24.4 in) in head-and-body length and weigh 7 to 12 kg (15 to 26 lb), with a maximum known mass of 15 kg (33 lb). The male has a red penis with a black scrotum.