Alouatta caraya are sexually dimorphic where males average 6.7 kg and females average 4.4 kg. Male body size ranges from 1.7 to 2.2 ft with tails of similar length to their body. Females' bodies average 1.6 ft with tails slightly longer than their bodies. A. caraya are also sexually dichromatic. Males usually have black hair, which gives the species the common name of Black Howler monkey.
Females however have more yellow-brown or olive colored hair.
Infants are born with a golden coat, which changes as the animal matures. A. caraya have long, strong prehensile tails. These tails are hairless on the underside, which allows them to be sensitive to touch and act in identifying things, much like a 5th hand. The black face is mostly hairless as well, with slightly bushy eyebrows. A. caraya have brown, medium sized eyes set in a frontal position.