Information systems (IS) projects involving multiple organizations are very common today. Knowledge integration in such projects is a complex task of integrating diverse knowledge bases across organizations that may possess distinct strategic goals and even conflicting interests. Prior research has indicated that social capital, a resource based on social relationships, positively influences knowledge integration and interorganizational relationships, but the exact nature of the interaction has been unclear. Based on an in-depth case study, this article examines a four-organization (three clients and one IT service provider) collaborative IS project wherein the clients were business partners for 7 years when they embarked on the project. The study explicitly identifies the roles through which social capital can be leveraged for knowledge integration in a collaborative IS project. Findings suggest that social capital can be leveraged as a motivator, an integrator, and a facilitator during the various stages of a collaborative IS project.