The proboscis monkey (Nasalis larvatus) was surveyed in the East Malaysian state of Sabah to establish its population status and to assess threats to its survival. It was found to be more widespread and abundant than previously thought, with a minimum population size of c. 5,907 individuals found along major coastal river systems in Sabah. The distribution of proboscis monkeys appeared highly fragmented, with only five major centers of continuous distribution and numerous small isolated populations. Existing proboscis monkey habitats are increasingly threatened by human activities. Of particular concern is the clearing and conversion of important riparian and coastal mangrove habitats to plantations and human habitation, which result in fragmentation of otherwise continuous populations along rivers, and local extinction of remnant populations trapped in small forest fragments. Only 15.3% of the population estimated in this study was found within protected forest reserves, with much of the species' diminishing range habitats exposed to further conversion, extraction and disturbance. Urgent mitigating strategies are necessary to ensure the long-term survival of proboscis monkeys in Sabah. Immediate actions are needed to prevent small isolated populations from local extinction, and long-term efforts must be undertaken to protect important proboscis monkey habitats and re-establish connectivity between fragmented populations.