The marmosets (), also known as zaris or sagoin, are 22 New World monkey species of the genera Callithrix, Cebuella, Callibella, and Mico. All four genera are part of the biological family Callitrichidae. The term "marmoset" is also used in reference to Goeldi's marmoset, Callimico goeldii, which is closely related.

Most marmosets are about 20 cm (8 in) long. Relative to other monkeys, they show some apparently primitive features; they have claws rather than nails, and tactile hairs on their wrists. They lack wisdom teeth, and their brain layout seems to be relatively primitive. Their body temperature is unusually variable, changing by up to 4°C (7°F) in a day. Marmosets are native to South America and have been found in Bolivia, Brazil, Colombia, Ecuador, Paraguay, and Peru. They have also been occasionally spotted in Central America and southern Mexico. They are sometimes kept as pets, though they have specific dietary and habitat needs that require consideration.

According to recent research, marmosets exhibit germline chimerism, which is not known to occur in nature in any primates other than callitrichids. 95% of marmoset fraternal twins trade blood through chorionic fusions, making them hematopoietic chimeras.