The implementation of socially sustainable supply chain management (SSCM) practices (i.e. assessment and collaboration) to tackle suppliers’ social deficiencies (e.g. the use of child labour) often requires a level of cooperation that can be difficult to establish. Despite this daunting challenge, scant scholarly attention has been paid to explore how the implementation of socially SSCM practices can be effectively facilitated and enhanced. Drawing on social capital theory, this study examines the individual impact of assessment and collaboration practices on suppliers’ social (...) performance and explores whether and how these effects can be moderated (strengthened) by the level of social capital (i.e. relational, cognitive, and structural) embedded in the buyer–supplier relationship. Based on a survey of 119 manufacturing companies in the UK, we found that assessment practices are less likely to influence suppliers to improve social performance compared to collaboration practices. However, when relational and structural capital are manifested in the relationship, assessment practices become significant in driving suppliers’ social performance. We also found that the positive impact of collaboration practices is more pronounced when relational and cognitive capital are established in the relationship. This paper contributes to the growing socially SSCM literature by disentangling the vital and relative importance of social capital dimensions on the implementation of socially SSCM practices. (shrink)