This study provides a direct test of social exchange theory on knowledge sharing from the perspective of the provider by examining the role of both perceived coworker support (PCS) and perceived organizational support (POS) on the extent to which employees share their knowledge with their coworkers. Also examined is the moderating role of knowledge tacitness. Results show PCS has a strong positive relationship with provider knowledge sharing but, contrary to expectation, POS does not have a significant relationship. Further, knowledge tacitness moderates the relationship between PCS and knowledge sharing such that the relationship between PCS and knowledge sharing is stronger for providers who perceive their knowledge as tacit. However, the difference in knowledge sharing between providers with knowledge high in tacitness versus low in tacitness is greatest at low levels of PCS and decreases as PCS increases. The implications of these findings to research and practice are discussed.