L. chrysopygus is covered almost completely in silky, dense black hair. The only place this differs is in the hind quarters. The thighs, buttocks, and base of the tail are colored a reddish-brown. This is what gives the animal its common name, golden-rumped lion tamarin. The face of L. chrysopygus is not haired, as well as the hands and feet, these are colored dark gray to black. The hind limbs are generally longer than the forelimbs and the tail is not prehensile. All digits have a pointed sickle shaped nail, which is used for gripping trees, except the big toe which has a flat nail. The dental formula is as follows, 2/2 incisors, 1/1 canines, 3/3 premolars, 2/2 molars. Body length in L. chrysopygus is between 20 cm and 33.5 cm, tail length is usually 31.5 to 40 cm, and mass ranges from 300 to 700 g. (Beacham 1998; Nowak 1999; Wolters 1990)
Range mass: 300 to 700 g.
Average mass: 572.5 g.
Range length: 20 to 33.5 cm.
Other Physical Features: endothermic ; bilateral symmetry