This paper introduces the Contextual SSCM Capability Framework (CSCM-CF), a new theoretical model that explains how sustainable supply chain management (SSCM) practices generate firm performance outcomes in agricultural and small-to-medium enterprise (SME) contexts in South East Asia (SEA). Despite growing empirical evidence on the SSCM-performance relationship, no integrative theory has adequately captured the unique institutional, economic, and cultural characteristics of SEA — a region that underpins some of the world’s most critical and sustainability-challenged agricultural supply chains. Grounded in the Resource-Based View (RBV), Stakeholder Theory, and the Triple Bottom Line (TBL) framework, the CSCMCF posits that SSCM capability is the key mediating construct between the adoption of SSCM practices and firm-level outcomes of financial performance, operational efficiency, and stakeholder trust. Seven theoretically grounded propositions are derived to direct future empirical research. The framework further identifies firm size, regulatory environment, industry sector, institutional capacity, and digital technology adoption as contextual moderators shaping the SSCM capability-development pathway in SEA. The CSCM-CF advances SSCM theory in emerging-market contexts and provides actionable guidance for agricultural supply chain managers, policymakers, and researchers.