Although negative out-group beliefs typically foster individuals’ motivation for collective action, we propose that such beliefs may diminish this motivation when people believe that this out-group cannot change in its very essence. Specifically, we tested the idea that believing in the malleability of immoral out-groups (i.e., targets of collective action) should increase collective action tendencies through group efficacy beliefs. Study 1 revealed that the more strongly participants believed that immoral out-groups could change as a function of contextual influences, the stronger their collective action tendencies were due to increased group efficacy. In Study 2, we experimentally replicated these findings using a manipulation of individuals’ beliefs about immoral out-groups being potentially malleable (vs. fixed). We discuss implications of our findings with an eye on the literature on collective action and implicit beliefs and on the promotion of civic engagement more broadly.