The functional use of a primate’s body greatly influences its skeletal morphology. Previous morphological studies include analyses of the glenohumeral joint as a reflection of the locomotion distinct to each Old World monkey. In addition, anthropologists examine the shoulder of hominoids to study the shift in locomotor behaviors towards bipedalism. The glenohumeral joint of hominoids, particularly those taking part in brachiation, is rounder and more globular in shape compared to other ancestral apes, indicative of a greater range in motion of the shoulder (Chan 2007a, 1; Green 2013, 253). However, little research examines the locomotion of New World monkeys, broadly described as arboreal quadrupedalism. In this study, I hypothesize that the morphology of the glenoid fossa and humeral head largely reflect the functional use of the shoulder. I examine the glenohumeral joint of 10 genera of Platyrrhini to assess whether Ateles, a brachiating New World monkey, displays the specific morphological features indicative of increased shoulder mobility. I also analyze the relative effects of phylogeny and body size on the variation in glenohumeral morphology. I calculate four indices, examining the breadth-length ratio of the humeral head and glenoid fossa, degree of curvature of the glenoid fossa and the location of the maximum glenoid breadth. The results of this study show that Ateles is distinct from other platyrrhine genera in having the roundest and widest humeral head and glenoid fossa. The unique morphology of the glenohumeral joint of Ateles compared to other New World monkeys suggests that there was a directional change related to an adaptation towards brachiation.