This paper examines the causal impact of appointing female directors to corporate boards on corporate social responsibility (CSR) performance, using Korea's 2022 gender quota as a quasi-natural experiment. Employing a Difference-in-Differences framework, we find that even a single female appointment to previously all-male boards significantly improves CSR performance, consistent with social role theory, group diversity perspective, and female risk aversion preference framework. Further analysis reveals that this effect is more pronounced among treated firms with weaker governance—characterized by low governance ratings and limited foreign institutional ownership—suggesting a governance-substitution mechanism. In contrast, Chaebol-affiliated firms, characterized by concentrated family control, show limited responsiveness, suggesting symbolic compliance in line with tokenism theory. Overall, our findings provide novel evidence on the effectiveness of board gender diversity and challenge the critical mass theory, which posits that at least three women are required for meaningful influence.