The Golden Lion Tamarin (Leontopithecus rosalia) is an endemic primate of the Brazilian Atlantic coastal rainforest, still under serious extinction risk. Just a few is know about its anatomy, specifically the muscle unions. Due to the reasons exposed before, aiming understanding the locomotion of this and others primates, we decided to study the morphology and morphometry of the gracilis and sartorius muscles, and the relation between them, in three Leontopithecus species. The current study examined 18 adult animal carcasses, of both sexes and with no physical abnormalities on the studied region. The material belongs to the Rio de Janeiro Primatology Center collection. The posterior members were dissected till the gracilis and sartorius muscles level, when were executed the morphometry, obtaining the cross section area and the histological analysis of the macroscopic between the gracilis muscle and the sartorius muscle. We described the morphology of the gracilis muscle and sartorius muscle, obtained the average muscle morphometry values and studied, histologically, the union between these muscles. The morphological and morphometric analysis allow us suggest descriptive parameters for these muscles the histological analysis allow us conclude the gracilis muscle and sartorius muscle fibers are not linked, but kept together by a conjunctive tissue, thus, being inserted into the medial side of the tibia. Functionally, we believe the gracilis muscle and the sartorius muscle shall contribute on the active containment of the knee joint and on the biomechanics of the posterior members, of these primates, known as runners