Drawing on perceived organizational support (POS) theory and employee-organizational relationship theories, this research investigated the association between POS and knowledge sharing as well as the potential moderating effects of perceived job security. Study participants were 255 information technology professionals and their supervisors working in the information technology industry in China. Findings showed that POS was positively related to knowledge sharing, and, as expected, perceived job security moderated the association. More specifically, the positive association between POS and employee knowledge sharing held only for employees who perceived higher job security from their organization. In contrast, POS was not significantly associated with knowledge sharing when employees perceived their job security to be relatively low. This latter result is consistent with contentions from employee-organizational relationships theories that limited investment by employers is likely to lead to lower contributions from employees. The findings are also congruent with arguments from social exchange theory that meaningful reciprocity is built on a history of open-ended exchanges whose development may be inconsistent with a shorter-term employment horizon.