As an institutional innovation aimed at enhancing corporate governance and protecting investor rights, the China Securities I nvestor Services Center (ISSC) serves as a platform facilitating engagement between minority shareholders and listed companies through active participation in shareholder meetings. Leveraging the ISSC’s shareholder activism as a quasi-natural experiment, this paper evaluates its impact on capital market information efficiency, focusing on both informational and governance effects. Empirical results show that ISSC engagement signifi cantly reduces stock price synchronicity, indicating improved information effi ciency. Further mechanism analysis reveals that this eff ect operates through enhanced accounting information quality and the suppression of tunneling activities by controlling shareholders. Heterogeneity analysis suggests that the improvement in information effi ciency is more pronounced in fi rms with greater investor attention, higher institutional ownership, and stronger analyst coverage. Overall, this study provides robust empirical evidence on the eff ectiveness of the ISSC’s shareholder activism and off ers policy implications for promoting investor protection and enhancing capital market effi ciency through interactive institutional arrangements.