Although South-east Asia harbours most of the world's tropical peatlands relatively little is known about the primate communities of the associated habitat, the peat swamp forest. To understand better the role of tropical peat swamp forests for the conservation of primates in general, and for the endemic primates of the Mentawai Islands in particular, we conducted a line transect survey in a 12.5-km2 section of peat swamp forest in northern Siberut. A total of 215 records of all four Siberut primates (Endangered Kloss's gibbon Hylobates klossii, Endangered Mentawai langur Presbytis potenziani, Vulnerable Siberut macaque Macaca siberu and Critically Endangered pig-tailed langur Simias concolor) were obtained. Pig-tailed langurs (65.5 km-2, 95% confidence interval, CI, 41.9–102.6) and Siberut macaques (35.8 km-2, 95% CI 25.5–50.4) were the most common species, with density estimates similar to (pig-tailed langur) or greater than (Siberut macaque) those in adjacent lowland rainforest on mineral soil. Density estimates of the Mentawai langur (2.7 km-2, 95% CI 1.3–5.3) and Kloss's gibbon (1.0 km-2, 95% CI 0.3–2.8) were approximately one-third and one-tenth, respectively, of the adjacent lowland rainforest. Given that resource density and diversity in peat swamp forest are probably lower than that of lowland rainforest, primate densities appear to be relatively high, with overall primate biomass (881 kg km-2) exceeding values for lowland rainforest on mineral soil. Our results underline the general importance that peat swamp forests may have for South-east Asian primates and for two island endemic species in particular.