ABSTRACT The growth of non-state actors has significantly changed the nature of conflict. While rebel groups are a persistently destabilizing force, private military and security companies (PMSCs) also increasingly enter conflict spaces on behalf of a variety of actors, including states seeking to suppress insurgencies. This case study of the Revolutionary United Front (RUF) during Sierra Leone’s civil war between 1991 and 2002 contributes to emerging work on rebel behavior by examining how rebel’s legitimacy seeking tactics evolve when PMSCs enter a conflict context. I explore the ways that PMSCs can shift insurgents’ interpretations of and engagements with legitimacy during conflict, thus fostering opportunities for shifts in rebel behavior. In Sierra Leone, PMSCs providing combat services which credibly threatened the RUF’s survival and status shifted incentive structures around legitimacy, providing the RUF the opportunity to engage in moral legitimacy-seeking tactics and alternative forms pragmatic legitimacy, emerging in the form of both coercive and cooperative control over civilian’s lives. While restricting some avenues of pragmatic legitimacy, this also engendered opportunities for substitution.