Although NGO-business relationships can bring many advantages for both the NGO and the commercial organization, research has shown that these relationships are still limited. While NGO-business relationships can easily be compared with buyer-supplier relationships in business supply chains, the literature specifically indicates several additional challenges in achieving effective and efficient NGO-business relationships compared to a commercial context. Our research seeks to understand how social capital can help overcome these additional challenges by creating sustainable NGO-business relationships. From a practitioner’s viewpoint, we not only strive to acknowledge the complementarity of NGOs and businesses for implementing sustainable relationship practices, but also seek to understand how these understudied cross-sector relationships can be successfully built. We use a multiple-case study design to investigate nine NGO-business relationships in a humanitarian context. This study contributes to the supply chain literature by demonstrating how social capital mitigates tensions within NGO-business relationships, enabling us to present a more generic process framework of enablers for creating sustainable NGO-business relationships.