The white–bellied spider monkey, Ateles belzebuth, is one of the seven species that inhabits both lowland and montane forests in the Peruvian Amazon. The lowland populations are locally extinct over a large part of the species range there, but there is very little information on the montane forest populations other than records of some groups in a few localities. The lack of information motivated us to conduct this study to determine the current status of these spider monkeys and identify the threats to their populations. Transect censuses were carried out in November 2016, February 2017 and May–July 2017. In 786 km of transects walked, we observed 44 groups of five primate species, the most common being A. belzebuth and Cebus yuracus (both with 13 groups). Most of the A. belzebuth groups were observed in Las Hamacas (six groups) and La Meseta (four groups) belonging to the Área de Conservación Privada Los Chilchos. Of the four species for which we obtained complete group counts, the largest groups were those of A. belzebuth (average 16 ± 6.1, n = 5) and the smallest of Alouatta seniculus (average 4.8 ± 1.2). The highest relative abundance was for A. belzebuth (2.56 individuals/10 km) and lowest for A. seniculus (0.53 individuals/10 km). South of the Río Marañón, the distribution of A. belzebuth is restricted to the montane forests of the regions of Amazonas, San Martín, La Libertad, and part of Huánuco to the Río Monzón, but it is locally extinct between the ríos Tocache and Monzón. Logging, hunting and deforestation for agriculture and cattle ranching were found to be the main threats to the survival of A. belzebuth and other primates in the areas we surveyed.