["Corporate Social Responsibility and Environmental Management, EarlyView. ", "\nABSTRACT\nThis study examines the nonlinear relationship between board gender diversity and corporate environmental sustainability disclosures in France. Drawing on critical mass theory, we apply a panel smooth transition regression (PSTR) model to 79 SBF‐listed companies over 2011–2019. Our results reveal an unusual transition modeling effect that occurs smoothly rather than abruptly. Specifically, we find that female board representation drives environmental disclosures only after reaching a critical threshold of three women (approximately 30% of board composition), supporting a U‐shaped rather than linear relationship. The study contributes the first application of PSTR to the gender diversity–environmental disclosure nexus and provides evidence‐based guidance for board committees, regulators, and ESG‐oriented investors.\n"]