The literature on knowledge sharing motivation has addressed the importance of both extrinsic and intrinsic motivation to sharing knowledge. However, a theory of knowledge sharing motivation in the public sector requires particular consideration since government’s main concern is in public service, not profits. Therefore, the present study introduces the concept of public service motivation (PSM) to the study of knowledge sharing among civil servants. Based on the PSM theory, this study postulates that civil servants’ interest in policy making, commitment to the public interest, compassion, and willingness for self-sacrifice can drive them to share knowledge for a sacred reason – serving the public interest. By using the data collected from middle-level public managers in Taiwan, the authors empirically tested whether PSM predicts knowledge sharing, and our hypotheses received strong support. Thus, PSM opens a new window for researchers interested in the study of knowledge sharing in the public sector. Points for practitioners: With the introduction of PSM, the present study connects knowledge sharing with public administration. Public service as a calling leads civil servants to share knowledge in order to create more advanced organizational knowledge and accordingly improve public service performance. Compared to situational factors (e.g. the use of information technology and rewards), PSM plays an even more pivotal role in promoting knowledge sharing, according to the results of our empirical research. Thus, altruistic motivation should be addressed and emphasized if knowledge sharing in the public sector is to be encouraged.