This study examines the effect of a firm’s ownership structure on its corporate social responsibility (CSR). Using firms that belong to Korean business groups, chaebols, as a sample and their ES ratings to measure CSR, we find a positive relationship between control-ownership disparity (i.e., a divergence between voting and cash-flow rights of controlling shareholders) and ES performance, especially social performance. Additionally, we show that when control-ownership disparities increase due to mergers between other affiliated firms, ES ratings rise significantly. Moreover, we find that firms controlled by descendants are more likely to exhibit higher levels of CSR compared to firms controlled by founders. Our results support the agency view of CSR, which suggests that CSR investment is associated with controlling shareholders’ incentives to pursue their own private benefits at the expense of minority shareholders.