To test whether the presence of a peer or a friend increases the motivation to be physically active in overweight and non-overweight youth in a laboratory setting. Youth motivation to be physically active as a function of the social context was measured using a computerized relative reinforcing value task to earn points exchangeable for physical and/or sedentary activities. The presence of a friend (p<.001) increased youth's; motivation to be physically active. The presence of a peer increased overweight youth's; motivation to be physically active, whereas this was not the case for lean youth (p=.47). Participants biked a greater distance in the presence of a friend than when alone (p<.001). Overweight youth biked a greater distance in the presence of a peer than when alone, while this was not the case for lean youth (p=.23). Friendships may increase youth's motivation to engage in physical activity and promote greater physical activity in non-overweight and overweight youth.