The species tends to have whitish to gray short 'trousers' rounded off by purplish-black faces with white sideburns.

it lacks the crest of tufted gray langurs and has hair on the crown no longer than on temples and nape.

[16][17][18][19][20][21] The purple-faced langur is found in closed canopy forests in Sri Lanka's mountains and the southwestern part of the country, known as the "wet zone".

Threats to this species include infringement on range by croplands, grazing, changing agriculture, road production, soil loss/erosion and deforestation, poisoning from prevention of crop raiding, and hunting for medicine and food.

[13][14] Its range has constricted greatly in the face of human encroachment, although it can still be seen in Sinharaja, Kitulgala, Kandalama, Mihintale, in the mountains at Horton Plains National Park or in the rainforest near the city of Galle.

While they normally avoid human habitations, fruit such as jak (Artocarpus heterophyllus), rambutan (Nephelium lappaceum), banana (Musa balbisiana), and mango (Mangifera indica) may contribute up to 50% to their diet in cultivated areas.

Purple-faced langur digestion is adapted to derive the majority of required nutrients and energy from complex carbohydrates found in leaves, with the help of specialized stomach bacteria.

[22] Where the species' diet is currently heavily dependent cultivated fruits, the ability to derive sufficient nutrition may become impaired.

[23] Semnopithecus vetulus feed on a less diverse diet than S. priam, with a greater proportion of leaves.

Food plants that have been identified include Holoptelea integrifolia, Hydnocarpus venenata, Macaranga peltata, Manilkara hexandra, Mikania scandens, Mischodon zeylanica, Pterospermum suberifolium, Tetrameles nudiflora, Vitex altissima, and Wrightia angustifolia.

For young or infirm langurs, black eagles, spot-bellied eagle-owls and possibly other larger birds of prey are perhaps not infrequently a threat.

Monitor lizards may also feed on purple-faced langurs (possibly through scavenging or picking off the young or sickly).

Rope bridges could be established for langurs to move between ranges safely, which may decrease the crossing of power lines and roads.

Replanting pine plantations with native species exploited by these langurs, could possibly increase its preferred habitat as well.

[14] Public education of conservation to the local people emphasizing compassion and kindness as well as explaining the importance and necessity of these mammals to the ecosystems overall biodiversity.