Virtual communities provide an important venue for knowledge sharing. Prior research has demonstrated that both system design factors, e.g., social presence, and social aspects of VC, e.g., social identity, are critical for encouraging knowledge contribution. However, we still lack a good understanding of how the system design and the social aspects of VC jointly influence members’ knowledge contribution. Also the uni-dimensional conceptualization of social presence in most prior research may not be sufficient to capture the complexity in VC interaction. To address these theoretical voids, a research model is developed to explain the effect of social presence on knowledge contribution as mediated through social identity. More particularly, drawing upon environmental psychology literature and prior research on social presence, we propose a three-dimensional conceptualization, consisting of sensory, affective, and cognitive components, and discuss their distinct roles in developing social identification and promoting knowledge contribution. The research model was empirically tested with a survey involving 430 registered members. The results provided a strong support for the validity and usability of the multi-dimensional conceptualization of social presence. We also found the distinct effects of various social presence dimensions on social identity and knowledge contribution. Both theoretical and practical implications are discussed.