The smallest monkey is the pygmy marmoset (Cebuella pygmaea). It weighs around 100 grams (3.5 ounces) and measures about 4.6 to 6.2 inches (11.7 to 15.8 cm) in length, excluding its tail. Native to the rainforests of the western Amazon Basin in South America, pygmy marmosets are known for their tiny size and distinctive vocalizations.