Squirrel monkeys are New World monkeys of the genus Saimiri. Saimiri is the only genus in the subfamily Saimiriinae. The name of the genus is of Tupi origin (sai-mirím or çai-mbirín, with sai meaning 'monkey' and mirím meaning 'small') and was also used as an English name by early researchers.

Squirrel monkeys live in the tropical forests of Central and South America in the canopy layer. Most species have parapatric or allopatric ranges in the Amazon, while S. oerstedii is found disjunctly in Costa Rica and Panama.

There are two main groups of squirrel monkeys recognized. They are differentiated based on the shape of the white coloration above the eyes. In total there are five recognized species. Squirrel monkeys have short and close fur colored black at the shoulders, yellow or orange fur along the back and extremities, and white on the face.

Squirrel monkeys have determined breeding seasons which involve large fluctuations in hormones and there is evidence of sexual dimorphism between males and females.

Squirrel monkeys can only sweat through the palms of their hands and feet. This can have the effect of making their hands and feet feel damp to the touch. Squirrel monkeys must make use of other thermoregulation techniques such as behavioral changes and urine washing. These monkeys live in habitats of high temperatures and high humidity, making it essential for them to maintain proper osmoregulation if conditions pass certain thresholds. Color vision studies have also been performed on squirrel monkeys for the purpose of better understanding vision ailments in humans.

The common squirrel monkey is commonly captured for the pet trade and for medical research, but it is not threatened. Two squirrel monkey species are endangered: the Central American squirrel monkey and the black squirrel monkey are listed as vulnerable by the IUCN.

Until 1984, all South American squirrel monkeys were considered part of a single widespread species, and many zoologists considered the Central American squirrel monkey to be a member of that single species as well. The two main groups currently recognized can be separated by the white above the eyes; it is shaped as a Gothic ("pointed") arch in the S. sciureus group, while it is shaped as a Roman ("rounded") arch in the S. boliviensis group. Mammal Species of the World (2005) recognized five species.

Subsequent taxonomic research has recognized Saimiri sciureus cassiquiarensis as a separate species Saimiri cassiquiarensis, and also recognized an additional species, Collins' squirrel monkey Saimiri collinsi that had previously been considered to be within S. sciureus. Some more recent taxonomies also recognize Saimiri sciureus macrodon as a separate species Saimiri macrodon, but others recognize S. macrodon to be a synonym of Saimiri cassiquiarensis.