Mechanically passive exoskeletons may be a practical and affordable solution to meet a growing clinical need for continuous, home-based movement assistance. We designed, fabricated, and preliminarily evaluated the performance of a wearable, passive, cam-driven shoulder exoskeleton (WPCSE) prototype. The novel feature of the WPCSE is a modular spring-cam-wheel module, which generates an assistive force that can be customized to compensate for any proportion of the shoulder elevation moment due to gravity. We performed a benchtop experiment to validate the mechanical output of the WPCSE against our theoretical model. We also conducted a pilot biomechanics study (eight able-bodied subjects) to quantify the effect of a WPCSE prototype on muscle activity and shoulder kinematics during three shoulder movements. The shoulder elevation moment produced by the spring-cam-wheel module alone closely matched the desired theoretical moment. However, when measured from the full WPCSE prototype, the moment was lower (up to 30%) during positive shoulder elevation and higher (up to 120%) during negative shoulder elevation compared to the theoretical moment, due primarily to friction. Even so, a WPCSE prototype, compensating for about 25% of the shoulder elevation moment due to gravity, showed a trend of reducing root-mean-square electromyogram magnitudes of several muscles crossing the shoulder during shoulder elevation and horizontal adduction/abduction movements. Our results also showed that the WPCSE did not constrain or impede shoulder movements during the tested movements. The results provide proof-of-concept evidence that our WPCSE can potentially assist shoulder movements against gravity.