How do rebel groups seek to legitimize themselves vis-à-vis the local population? The dimension of legitimacy and the legitimation strategies of rebel groups have long been understudied topics in scholarship on rebel governance and in conflict studies at large. Nonetheless, in recent years, an increasing number of works have sought to address these aspects, from both a theoretical and empirical perspective. In particular, Schlichte and Schneckener observed that such groups might resort to a range of “sources of legitimacy” to legitimize themselves—both symbolically and performatively—in the eyes of different audiences, including the local audience. The paper seeks to contribute to research on the topic by analyzing the legitimation strategies employed by different militant Islamist groups in Syria vis-à-vis the local population. It will focus on Hay’at Tahrir al-Sham (former affiliate of al-Qa‘ida); Ahrar al-Sham (part of the National Liberation Front); the so-called Islamic State; and Tanzim Hurras al-Din (comprising al-Qa‘ida loyalists). The analysis draws on different sources—mainly official documents and media production by the groups under consideration, as well local news reports, ideally complementing them with interviews and conversations with experts, local observers, and community members. The paper investigates inter-group differences and the variance of legitimation strategies over time. Besides this, it tries to understand how legitimacy building relates to the groups’ modes of governance versus lack of engagement in governance. In doing so, it argues that legitimacy building is relational and can be subject to change, and it is closely connected to the trajectories of such groups at large. Findings could contribute to further illuminating processes of legitimacy building in rebel groups and non-state armed actors at large.