Corporate sustainability is a dynamic, socially constructed concept. Relatedly, to understand the variations in the disclosure of corporate sustainability activities across countries, we need to inherently explore their underlying sociopolitical contexts. At present our understanding in this regard, is deficient. We respond to this extant research gap by adopting a multi-country approach to investigate the relationship between countries' institutional environments and firms' sustainability disclosure (SD) practices, across six countries in the Southeast Asian region. Our findings reflect a common focus of Southeast Asian firms on community and human resources (HRs) related disclosures. Nevertheless, nuanced differences in their overall SD levels confirm the influence of differing legal, normative, and sociocultural systems in engendering greater disclosure and transparency at a national level. By quantifying the institutional environment and identifying external influencing factors, our study provides a useful framework grounded in neo-institutional theory to widen the existing understanding of how institutional pressures can be measured and compared across different contexts.