Fossil hominin bipedality is thought to have evolved as an adaptation from arboreality, and extant primate models of the dynamic relationships between both forms of locomotion can improve our understanding of our earliest ancestors. Hylobatids (gibbons and siamang), whose locomotor repertoires are primarily characterized by brachiation, are the most frequently bipedal ape besides humans, but little is known of how locomotor signals develop in their hindlimb joint and diaphyseal morphology. This study tests ontogenetic scaling of locomotor signals in the knee joint and external diaphyses of white-handed gibbons (Hylobates lar). Caliper measurements on the femur, tibia, and patella along with age, body mass, and articular surface areas were compared in scaling relationships using allometric regressions. All knee joint articular surfaces were negatively allometric with body mass, but the medial condylar areas of both the tibia and femur had higher slopes relative to lateral condylar areas of both the tibia and femur. The hylobatid knee indicates that bony adaptations to both arboreality and bipedality appear early in development and that these adaptations can facilitate both arboreality and bipedalism.