Adult Roosmalens' dwarf marmosets have a total length of 38–39 cm (15-15½ in), incl. a tail of 22–24 cm (8½-9½ in), and weigh 150-185 g (5½-6½ oz). It is the second smallest species of monkey, with only the related pygmy marmoset being smaller. The upperparts of Roosmalens' dwarf marmoset are mainly dark olive-brown, while the underparts are pale, dull yellowish.