This study addresses a research gap in corporate governance and sustainability by examining the influence of women on boards on sustainability performance, with a specific focus on the moderating role of national culture in the banking sector. We analyze data from 641 publicly listed banks across 63 countries from 2011 to 2023. Employing an unbalanced panel data approach with fixed-effects model and a series of robustness tests, this study finds that WOB significantly impact sustainability performance, particularly when a critical mass is reached. Moreover, the positive effect of WOB is more pronounced in countries with high power distance, collective social values, femininity, low uncertainty avoidance, high long-term orientation, and a restraint-oriented cultural framework. By exploring the moderating role of national culture, this study contributes to the growing literature on gender diversity and sustainability. The findings offer both theoretical and practical insights to enhance sustainability practices in the banking sector. Particularly, they provide valuable guidance for banks, policymakers, and stakeholders on leveraging board gender diversity to improve sustainability outcomes. Additionally, the study highlights the importance of tailoring sustainability strategies to specific cultural contexts, emphasizing the critical role of gender diversity in corporate governance.