Drawing on social capital theory, we investigated how shared goals affect the intentions of knowledge sharing and knowledge withholding across professional boundaries mediated through exploration activities. To examine the model, we performed structural equation modelling using two sets of two-wave survey data on nurses from two acute-care hospitals in Japan (Study 1, n = 178; Study 2, n = 210). The results of both studies showed that shared goals had an indirect positive effect on knowledge-sharing intention and an indirect negative effect on knowledge-withholding intention, fully mediated through exploration activities. The present study contributes to the literature by identifying the full mediation effects of exploration activities in linking shared goals to knowledge sharing/withholding across boundaries. This study sheds light on the positive effects of exploration activities in overcoming knowledge-sharing barriers across boundaries.