This article examines how the civilian constituencies of rebel groups affect their use of violence against civilians. While past research has acknowledged the importance of rebel constituencies, they are primarily seen as only having an indirect effect on rebel behavior. In this study, I conceptualize rebel constituencies as central political opportunity structures for rebel groups providing incentives and imposing restraints on their use of strategic violence and the violent behavior of individual rebel fighters. In particular, I hypothesize that a constituency overlap between rebels and the government of a state acts as a restraint making large-scale violence against civilians less likely. In contrast, high levels of constituency fractionalization and polarization induce strategic violence and predatory behavior, increasing the chances of large-scale civilian victimization. I conduct a statistical analysis of rebel one-sided violence in sub-Saharan Africa using newly collected data on rebel constituencies to test these hypotheses. The results only provide limited empirical support for the hypothesized relationship between constituency overlap and rebel violence against civilians. There is clear empirical evidence, however, that heavily fractionalized and polarized rebel constituencies are associated with higher levels of violence against civilians.