The weight is as much as 8 kg for captives, but it is probably nearer 6 kg for animals in the wild. Most forms have coarse, stringy hair, but some have a soft and fine pelage. Underfur is lacking.
Most forms have a black face with white eye rings, but some forms have a flesh-colored face.
Spider monkeys have an exceptionally long tail and legs in relation to body length, a prehensile and extremely flexible tail, and thumbs that are poorly developed or lacking. The head is small, and the muzzle prominent. The clitoris of the female is greatly elongated, so that it is often mistaken for the penis of the male. Spider monkeys are found in rain and montane forests and tend to occupy small branches of the high strata of the canopy. Probably only the gibbons exceed Ateles in agility in trees.