Social capital is an important ecosystem service, yet we lack common understanding of how it fits, and can be operationalized, within the ecosystem services framework. We review the literature to clarify the role of social capital in this context, establishing it as a multidimensional concept and a fundamental constituent of human well-being that is both supported by, and affects, all categories of ecosystem services. We then draw on qualitative and quantitative data to assess and value social capital as an ecosystem service and explore its role in facilitating management goals in a Malagasy locally managed marine area. We find high levels of social capital, gauged by trust, community involvement, and social cohesion. Results of a choice experiment show positive utilities associated with high levels of social cohesion. Respondents also ranked social cohesion higher than some provisioning, regulating, and cultural ecosystem services. Qualitative data suggest social capital increased as a result of the community based management institution, and has facilitated the success of marine management measures. Our results offer insight into the ways in which social capital can both affect, and be affected by, the management of natural resources, and how it can be assessed and valued as an ecosystem service.