How do non-state armed groups form in intra-state armed conflicts? Researchers have started to disaggregate armed groups, but we still know little about how armed groups emerge in different ways. Drawing on the literature on social movements, civil wars, and civil–military relations, we generate a typology of ‘movement’, ‘insurgent’, and ‘state splinter’ origins of armed groups. We argue that fundamentally different dynamics of conflict shape armed group origins in the context of broad-based mobilisation, peripheral challenges to the state, and intra-regime fragmentation. Armed groups that emerge in these contexts in general differ in their initial membership and leadership, the basic organisational dimensions that we focus on. We demonstrate the utility of our typology by mapping different origins of armed groups onto existing cross-national data and charting type narratives in illustrative cases. This discussion advances recent efforts to understand the importance of armed group emergence for outcomes of interest to conflict scholars by moving beyond either separate types of origins or highly disaggregated organisational analyses to broader conflict dynamics through which armed groups form, with implications for how these groups act. Future research should consider different origins which we identify in comparison through an in-depth analysis of armed groups’ complex histories.