Boundaries between project team and other units emerge when information system development project team is formed. Difficulties, during project execution, introduced by such boundaries highlight the importance of boundaries spanning for project team. On the other hand, previous studies have highlighted the critical role of social capital on performance, through enhancing acquisition, exchange, and integration of intellectual resources. While the impact of social capital on teamwork performance has been illustrated by past studies, in this study, we move further and argue that social capital contributes to project performance through promoting boundaries spanning. We first identify three critical boundaries between information system development (ISD) project team and user group, and then validate the mediating role of boundaries spanning, between social capital and project performance. Data collected from 184 practitioners confirmed the proposed concept.