The pygmy marmoset (Cebuella pygmaea) is a small New World monkey native to the rainforests of the Amazon Basin in Brazil, Colombia, Ecuador, Peru, and Bolivia. It is notable for being the smallest monkey in the world at just over 100 grams (3.5 oz). It is generally found in evergreen and river edge forests and is a gum-feeding specialist.

The pygmy marmoset lives in groups of five to nine individuals with a dominant male and a breeding female. The communication system is complex and includes vocal, chemical, and visual signals. There are three main types of call which depend on distance the call needs to travel and visual displays may be made when the monkey is threatened or to show dominance. Chemical signalling using secretions from glands on the chest and genital area allow the female to indicate to the male when she is able to reproduce. The female gives birth to twins twice a year and the parental care is shared between the group.

The pygmy marmoset has been viewed as somewhat different from typical marmosets, most of which are classified in the genera Callithrix and Mico, and thus is accorded its own genus, Cebuella, within the family Callitrichidae. It is listed as Least Concern by the International Union for Conservation of Nature as it is common across its wide range and not at immediate risk of widespread decline. It is sometimes threatened by habitat loss and the pet trade.

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