Disproportionately long, spindly limbs inspired the spider monkey's common name. Their deftly prehensile tails, which may measure up to 89 cm (35 inches), have highly flexible hairless tips complete with skin grooves similar to fingerprints; this adaptation to the spider monkey's strictly arboreal lifestyle grants the monkeys a fifth hand of sorts. Adults reach an average body length of 50 cm (20 inches) and a weight of 6.4 kilograms (14 pounds).

Their arms are very small, but yet very long. With legs that are on the shorter end, the monkey walks with its arms practically dragging on the ground. However, they are one of the very few kinds of monkeys that don’t rely on their arms to help them walk. They just stand and walk on 2 feet. If some trouble occurs as far as balance goes they just grab onto their tale to help them. Their hands resemble their arms, being long and narrow. Another feature to the monkey that makes it recognizable is that its nostrils are very far apart.

Spider monkeys have hook-like, narrow and thumbless hands; the fingers are elongated and recurved. The hair is coarse, ranging in colour from a ruddy gold to brown and black; the hands and feet are usually black. Heads are small with hairless faces. An unusually long labia in females may be mistaken for a penis; its function is unclear.

There is speculation that the alleged Loys's Ape is actually a large spider monkey, but this is still a matter of intense debate amongst cryptozoologists.