Advantaged allies’ support of disadvantaged groups’ struggle for equality constitutes an important prerequisite for lasting social change. We conducted three studies in two national contexts (Poland, U.S.) to test whether zero-sum beliefs moderate advantaged group members’ support for solidarity-based and system-supporting collective action. Study 1 (N=1,019) showed that zero-sum beliefs were positively related to system-supporting collective action and negatively related to system-challenging collective action. Study 2 (N=312) replicated this effect and demonstrated that an advantaged group (White Americans) who moderately or strongly identified with their racial ingroup endorsed zero-sum beliefs more than a disadvantaged group (Black Americans). In Study 3 White Americans (N=494) experienced heightened levels of anger and fear when reminded of the growing proportion of racial minorities in the U.S. population (relative to a control condition). Greater fear (but not anger), in turn, translated into lower support for system-challenging and higher support for system-supporting collective action. These indirect effects were only significant among White Americans who reported moderate and high levels of zero-sum beliefs. Overall, our results provide strong empirical support for the role of perceived competition between groups in society in shaping advantaged group members’ support for system-supporting and system-challenging collective action.