This study examines the impact of board gender diversity on firm performance in Japan, using data from 1990 publicly listed companies from 2006 to 2023. The findings, derived from a fixed-effects regression model, indicate a significant negative correlation between board gender diversity and firm performance. This adverse effect is more pronounced in smaller firms, those with higher leverage, reduced institutional ownership, and regulated and consumer-focused industries, particularly before the COVID-19 pandemic. The detrimental impact is transmitted through corporate social responsibility and firm innovation, rather than board independence or CEO duality. The study also addresses potential endogeneity using a two-stage least squares (2SLS) regression model, employing an equal opportunity policy as an instrumental variable. The results are robust when using return on equity (ROE) as a performance metric. The analysis does not reveal a nonlinear relationship between board gender diversity and corporate performance. The study contributes to the literature on corporate governance in Japan, highlighting the need for further exploration of how firm size, leverage, institutional ownership, and sector classification moderate the impact of gender diversity on firm performance.This study examines the impact of board gender diversity on firm performance in Japan, using data from 1990 publicly listed companies from 2006 to 2023. The findings, derived from a fixed-effects regression model, indicate a significant negative correlation between board gender diversity and firm performance. This adverse effect is more pronounced in smaller firms, those with higher leverage, reduced institutional ownership, and regulated and consumer-focused industries, particularly before the COVID-19 pandemic. The detrimental impact is transmitted through corporate social responsibility and firm innovation, rather than board independence or CEO duality. The study also addresses potential endogeneity using a two-stage least squares (2SLS) regression model, employing an equal opportunity policy as an instrumental variable. The results are robust when using return on equity (ROE) as a performance metric. The analysis does not reveal a nonlinear relationship between board gender diversity and corporate performance. The study contributes to the literature on corporate governance in Japan, highlighting the need for further exploration of how firm size, leverage, institutional ownership, and sector classification moderate the impact of gender diversity on firm performance.