In this study, we examine the impact of female co-opted directors on investment efficiency. Using 14,856 US firm-year observations from 1998 to 2023, we find a positive association between female co-opted directors and investment efficiency. Additionally, analysis reveals that the effect of female co-opted directors in improving investment efficiency is pronounced for companies with lower levels of institutional ownership and weaker corporate governance. Our results remain consistent across multiple robustness checks and identification strategies, indicating that female co-opted directors enhance investment efficiency by alleviating agency problems. Our study has important implications for corporate managers, shareholders, and regulators, as it highlights the role of female co-opted directors in enhancing managerial accountability and investment efficiency. It provides policy implications for regulators against shareholder pressure on firms to mitigate co-option-agency issues and implement efficient investment decisions.