Black-headed night monkeys are about the same size as squirrels. Their small but sturdy bodies are covered in short and thick fur. Adults have a dark gray and black agouti pelage on their upper back and forearms. The lower back and outer rear legs are tinted with orange-tan fur. Orange fur is also present on the sides of the neck, lower neck, and belly. Their tails have orange and gray fur throughout.

The name “black-headed” comes from the three conspicuous black stripes on their forehead. Two of these stripes are present on the side of the face to the maxilla (the upper jawbone). The third and central stripe extends from the forehead to the bridge of the nose. A small patch of white extends over each eye, on the cheeks, and under the chin.

Hidden behind their dense fur are tiny ears that just barely peek out of either side of their head. In fact, the scientific name Aotus means “earless.” However, do not let their ear size fool you, because their hearing is superb—they can pick up the sounds of even the slightest movements.

The eyes of the black-headed night monkey may look disproportionately large to the size of their head. The size of their eyes improves their nocturnal vision, which increases their ability to be active at night. Unlike many nocturnal animals, the black-headed night monkey lacks a tapetum lucidem in the eye, which is a reflective layer of tissue behind the retina. This gives nocturnal animals the typical “eyeshine,” like a cat. In addition, their vision is monochromatic, in which they only see in one color, typically shades of gray.

Their hind limbs are slightly longer than their forelimbs. Mainly quadrupeds, their strong hind limbs make them adept leapers.

In addition, black-headed night monkeys have long slender digits with flat nails and thick fingertip pads. The second digit on each foot has a long curved nail, known as a toilet claw, for grooming. Their long, nimble fingers enable them to grasp onto tree trunks and branches while foraging.