Purpose Drawing on signaling theory and social capital theory, this study aims to examine the underlying mechanisms and contingencies of the relationships between social capital (SC) and collaboration in buyer-supplier relationships (BSRs). This is achieved by evaluating the mediating effect of psychological contract fulfillment (PCF) and the moderating roles of guanxi orientation and market uncertainty. Design/methodology/approach The current study used a survey method to collect data from 271 buyers in China. Structural equation modeling (SEM) and moderated regression analysis were applied to examine the hypotheses. Findings The positive effect of structural and relational SC on buyers’ collaborative behaviors is partially mediated by buyers’ PCF. In contrast, the positive effect of cognitive SC on collaboration is fully mediated by buyers’ PCF. Guanxi orientation strengthens the indirect effect of buyers’ PCF on the cognitive SC-collaboration relationship and relational SC-collaboration relationship. Market uncertainty amplifies the relational SC-collaboration relationship. Originality/value Prior studies have presented mixed evidence of the effect of SC on collaboration and have paid little attention to the underlying mechanisms and conditions moderating the effect. This research proposes a theoretical model that integrates signaling theory and social capital theory to explore how three dimensions of SC can enhance buyer collaboration through buyers’ PCF under different levels of guanxi orientation and market uncertainty.