--- - |2 Abstract Social identity, shared grievances, and group efficacy beliefs are well‐known antecedents to collective action, but existing research overlooks the fact that collective action often involves a confrontation between those who are motivated to defend the status quo and those who seek to challenge it. Using nationally representative data from New Zealand (Study 1; N = 16,147) and a large online sample from the United States (Study 2; N = 1,513), we address this oversight and demonstrate that system justification is negatively associated with system‐challenging collective action, but positively associated with system‐supporting collective action, for members of both low‐status and high‐status groups. Group identification, group‐based injustice, group‐based anger, and system‐based dissatisfaction/anger mediated these relationships. These findings constitute the first empirical integration of system justification theory into a model of collective action that explains when people will act collectively to challenge—and, just as importantly, defend—the status quo. - European Journal of Social Psychology, EarlyView.