As modern business competition increasingly takes the form of competition among supply chains, supply chain capability has come to be recognized as an important source of competitive advantage for firms. Many companies are gradually transitioning from market transaction-based approaches to collaboration-based approaches in supply chain management. As concerns have intensified over the potential weakening of overall manufacturing competitiveness resulting from the polarization between large corporations and small and medium-sized suppliers, interest in win-win cooperation between large enterprises and small and medium-sized suppliers has steadily grown in Korea over the past decade. However, domestically, win-win cooperation has remained at the level of slogans and declarations, failing to achieve practical and developmental implementation, and has been insufficient in forming a theoretical academic framework. This study utilized social capital theory to explain how win-win cooperation positively influences supply chain performance and empirically analyzed this relationship using Korean firms. First, we identified the concept and components of win-win cooperation and proposed a theoretical model in which win-win cooperation contributes to the formation of inter-firm relational capital and, through this, to performance improvement. Statistical analysis of 197 data points collected from small and medium-sized suppliers supported the hypotheses that positive relationships exist among win-win cooperation (fair trade and cooperative activities), relational social capital, and supplier production and operations performance. Furthermore, by confirming the full mediating effect of relational social capital, this study revealed that win-win cooperation contributes to improving supplier production performance through the mechanism of enhancing the accumulation of relational social capital. This study provides implications not only for purchasing large enterprises and managers of small and medium-sized suppliers but also for government policymakers seeking to strengthen the overall capabilities of manufacturing supply chains through win-win cooperation.