Macaca pagensis formerly included the overall darker Siberut macaque as a subspecies, but this arrangement is polyphyletic,[3] leading to the two being classified as separate species.

Their backs have a dark brown coloration, and chestnut to pale ochre on the sides of the neck, the front of the shoulders and the undersides of this species.

Roaming, solitary Pagai Island macaques may challenge the dominant male for his position, leading to aggressive fights.

Females show fertility and willingness to mate by displaying their swollen and reddened genitals.

The species' primary habitat is on the Mentawai Islands 150 km (93 mi) off the west coast of Sumatra.

The primary reasons behind deforestation on the island are the clearing of large areas of land for cash crop and oil palm plantations, as well as commercial logging.

The alternating flooding and low water levels has also caused an increase in the population of malarial mosquitoes.