Northern pig-tailed macaques are large, thick-set monkeys with an olive-gray pelage that forms a darker brown crown on their heads. When standing quadrupedally, their broad chest and comparatively long legs mean they have an almost box-like shape. They have pinkish faces with patches of pale skin around the eyes and distinctive red lines extending from the outside corners of the eyes to the edge of the face. They have long hairs on their cheeks (the cheek ruff), which are especially prominent in adult males. Possibly their most distinctive feature is their short, thick tail, which is often arched over their backs. Their genitals and hindquarters are a darker red. Babies are born a darker brown and become lighter as they age. Compared to the southern pig-tailed macaque, northern pig-tailed macaques have a grayer pelage and more pronounced cheek ruffs.