The Guiana Shield has large pristine tracts of tropical forest with high biological diversity and is an area of endemism within the Amazon Basin. However, the conservation status of primates in eastern Amazonian Brazil is still poorly known. Here, we report information on relative abundance, group size, density estimates, plus the effects of environmental variables and seasonality of primates in a sustainable-use reserve in the eastern Brazilian Amazon. From a 603 km transect-based census conducted in October–December 2013 and March–June 2014 we obtained 122 sighting records of six primate species. The most common were Ateles paniscus (45 detections) and Saguinus midas (40 detections). The high calculated density estimates for Sg. midas (2.01 groups/km2 or 12.05 individuals/km2) and for At. paniscus (3.44 groups/km2 or 10.31 individuals/km2) underscore the conservation importance of the study area for the vulnerable At. paniscus. We found no effect of environmental variables on the number of detections of primates, except for At. paniscus and Cebus olivaceus, with the former showing a higher number of detections in more open canopy forest during the rainy season, and the later having a higher number of detections in areas with higher density of palms in the dry season.