AbstractResearch Question/IssueThis study examines activism campaigns where multiple activists simultaneously target the same firm—which we term clustered shareholder activism. Despite the growing influence of shareholder activism on corporate governance, the clustered activism phenomenon has previously only been addressed indirectly, anecdotally, or with limited data. We consider cost sharing motives for clustered activism and whether the phenomenon exerts a positive or negative impact on the performance of the target firm.Research Findings/InsightsUsing a large dataset of shareholder activism events at U.S. firms, we find that clustered activism campaigns are more common at larger firms and among geographically proximate activists, which is consistent with our prediction that activists cluster to reduce the costs associated with activism campaigns. Furthermore, we find that clustered activism produces elevated profitability and abnormal returns, which is consistent with our prediction that activists cluster to address principal–agency costs.Theoretical/Academic ImplicationsOur study provides some of the first theoretical and empirical evidence on the clustered activism phenomenon. We contribute to the understanding of the role of shareholder activism by considering their effect on principal–agency and principal–principal problems. Our results also contribute to the literature that examines factors relating to the success of shareholder activism by documenting the effect of clustered activism on activism costs and target firm performance.Practitioner/Policy ImplicationsOur study adds to the debate among practitioners and regulators on the merits (or lack thereof) of clustered activism. Our findings suggest that a regulatory approach that encourages clustered activism can benefit shareholders.Video Abstracthttps://onlinelibrary.wiley.com/page/journal/14678683/homepage/videoabstracts.htmlyoutubehttps://www.youtube.com/watch?time_continue=1&v=0_D-6Pw9sYo