<p>Diversified certification (e.g. management systems, products, and service certifications) has become a critical strategic tool for firms seeking to enter new markets and meet supply chain and customer requirements. Drawing on resource orchestration and signalling theories, this study develops a theoretical framework to explore how diverse certification affects corporate Environmental, Social, and Governance (ESG) performance through a dual-path mechanism involving internal resource integration and external signal transmission. Based on panel data from Chinese listed small- and medium-sized enterprises (SMEs) from 2014 to 2023, the empirical analysis reveals that diversified certification significantly enhances ESG performance, confirming the theoretical expectations. Further analysis shows that the higher a firm’s internationalisation level, the stronger the positive effect of certification on sustainable performance. Moreover, media attention exhibits a significant dual mediating effect on the relationship between certification and ESG performance, and this mediating mechanism is nonlinearly moderated by the degree of internationalisation. This study reveals how diversified certification improves sustainable development by enhancing internal resource management and increasing external recognition. The findings provide theoretical support and practical insights for SMEs in emerging markets to formulate certification strategies and enhance international sustainable competitiveness.</p>