Mico humeralifer (É. Geoffroy Saint-Hilaire, 1812) is a callitrichid monkey commonly called the Santarém marmoset or black-and-white tassel-eared marmoset. It is a small (280–310 g), sexually monomorphic anthropoid primate with a diet of insects, fruits, and plant exudates (nectar and gum). It is endemic to northern Brazil, occurring in the states of Pará and Amazonas, south of the Rio Amazonas, along the western margin of the Rio Tapajós in dense ombrophilous forests of the Amazon. M. humeralifer is the smallest primate in its distribution range. It lives in small groups and inhabits primarily terra firme forested regions in the Amazon Basin, adapting quickly to second-growth forests. Due to the lack of knowledge about its demographics and its remote habitat, its current conservation status is “Data Deficient.”