Head and body length of S. sciureus is 260-360 mm, tail length is 350-425 mm, weight ranges from 750-1,100 grams. The short, thick, soft fur is brightly colored and there is a circular patch around the lips and nostrils which is hairless. Coloration varies, but most commonly the markings are as follows: white around the eyes extending behind the ears, down the sides of the throat and neck; gray to black on the top of the head, reddish or yellow forearms, hands and feet with the shoulders and hind feet touched with gray; underparts white to ochre; and the tail colored similarly to the body, but with the terminal portion black. Coloration and markings are similar in all ages and both sexes. The large eyes are closely set and the ears are shaped like human ears. S. sciureus has a short but well-developed thumb and a long tail which is not prehensile.

Common Squirrel Monkeys are found in both primary and secondary forests and cultivated areas preferring habitats with streams. They are active in day and night. They are considered both arboreal and diurnal animals. The species feeds on insects found in its habitat. However, the Common Squirrel Monkey is not an absolute insectivore. They are also frugivorous animals. Their diet includes various fruits and berries. It has been established that sometimes they follow Cebus monkeys to find fruits. It is interesting to know that Common Squirrel Monkeys can also feed on molluscs, frogs, and small vertebrates. They often inhabit areas near water and yet they can obtain most of their water from their meals. Sometimes, Squirrel Monkeys drink water from tree holes and on the ground.

They do appear to be particularly territorial. This primate forms larger groups than any other New World Monkeys. Social subgroups are often comprised of pregnant females, females with young, and adult males. The mating season reveals and hierarchy achieved by fierce fighting, and the dominant males interacting with the females. Once the females give birth they form a subgroup which excludes the males and likely has a hierarchy of its own. Another subgroup of young males and preadult males, has individuals who leave to join the adult males as they reach sexual maturity, sometimes being required to fight to win a place in this group. S. sciureus is among the most vocal of all primates using chirps and squeaks for alarm, squawks and purrs during mating and birth seasons, barks of aggression and screams when in pain. 26 distinct calls have been recorded in these monkeys.

Reproductive data is available for a captive population and indicates a 2-4 month mating period during the dry season. Births occurred between late June and early August, gestation lasting from 152-172 days. At birth newborns weigh about 100 grams and cling to their mothers back for the first few weeks. At 1 year of age they are independent, females reach sexual maturity at 3 years and males at about 5.

S. sciureus is found in N Brazil north of the Amazon-Jurua system, and south of the Amazon, east of the Rio Xingu or the Rio Iriri; Marajo Isl (Brazil), Guyana, French Guiana, Surinam, Venezuela, Columbia, E Ecuador, and NE Peru.