This study investigated the effect of trust, motivation, and reward on the willingness to share knowledge on social networks by the Shiraz University of Medical Sciences (SUMS) staff. The instrument of the present study was a researcher-made questionnaire in which formal-content validity was confirmed, and its reliability, according to Cranach's alpha coefficient, was equal to .90. The study's statistical population was 27506, including employees and faculty members, according to the Krejcie and Morgan table, the sample size was 379. The results showed that the importance of knowledge sharing among employees and commitment to the university were two effective components of intrinsic motivation. Support of university administrators for knowledge sharing and improving the status of social pages (38.8%) through knowledge sharing were two effective components of external motivation for employee knowledge sharing in social networks. "Fear of employees being replaced" and "fear of losing a job" were two negative and very weak components of the trust variable on knowledge sharing. "Financial incentives" and "job promotion" were among the influential components of the reward variable, which "greatly" affected the sharing of knowledge in social networks. Paying more attention to evaluating the performance of employees and financial rewards by managers can be effective in sharing knowledge and motivation.