Despite the recognized importance of board gender diversity and eco-innovation, it is far from clear how board gender diversity can improve eco-innovation. Grounded in upper echelon theory and the gender socialization theory, we suggest that this relationship operates through corporate social responsibility (CSR) strategy. In a sample of FTSE 350 firms, we find that CSR strategy mediates the influence of the board's gender diversity on eco-innovation. These findings are robust to several regression specifications and alternative samples. This work offers empirical evidence that gender diversity on boards can play a significant role in promoting firm's CSR strategy, which in turn improves eco-innovation. Our findings could provide useful insights for regulators and stakeholders concerned with enhancing corporate governance structure and promoting eco-innovation.