This study examines the impact of board gender diversity on ESG performance. According to resource dependence theory, increasing gender diversity on corporate boards allows for diverse perspectives and independent opinions, which can have a positive impact on companies. Additionally, according to institutional theory, companies seeking social legitimacy through gender diversity may explicitly set goals to secure female executives, and such companies that prioritize legitimacy are known to actively engage in activities related to sustainable management. Based on these discussions, it is expected that board gender diversity will have a positive impact on improving ESG performance, and empirical analysis was conducted to test this hypothesis. For empirical analysis, a sample of 6,336 firm-year observations listed in the KOSPI from 2019 to 2021 was used. Using ESG ratings provided by the Korea Institute of Corporate Governance and Sustainability, empirical analysis revealed a significant positive association between board gender diversity and ESG performance. Furthermore, significant results were found when analyzing individual ESG indicators. These results contribute to the understanding of the direct impact of board gender diversity on ESG ratings, which are used to evaluate a company's sustainability. It is also meaningful in expanding the theoretical discussion by discovering that companies that value the legitimacy granted by stakeholders are relatively more likely to pursue gender diversity and sustainability.