Squirrel monkey fur is short and close, colored olive at the shoulders and yellowish orange on its back and extremities. Their throat and the ears are white and their mouths are black. The upper part of their head is hairy. This black and white face gives them their German name, "skull monkeys".

Squirrel monkeys grow to 25 to 35 cm, plus a 35 to 42 cm tail. They weigh 750 to 1100g. Remarkably, the brain mass to body mass ratio for squirrel monkeys is 1:17, which gives them the largest brain, proportionately, of all the primates. Humans have a 1:35 ratio.