This study provides a direct test of social exchange theory to knowledge sharing from the provider perspective by examining the effects of both perceived organizational support (POS) and perceived coworker support (PCS) on their direct knowledge sharing with their coworkers. Also examined is the moderating effect of providers’ perception of the tacitness of their knowledge on the relationship between these different sources of support and their knowledge sharing. Results show perceived coworker support (PCS) has a significant positive effect on provider knowledge sharing but that the relationship only holds when knowledge tacitness is high. Contrary to expectations, perceived organizational support (POS) did not have a significant positive effect on providers’ knowledge sharing and that this relationship did not depend on providers’ perceptions of their knowledge tacitness. The implications of these findings to research and practice are discussed.