International criminal justice can have intended and unintended impact on the legitimacy of quasi-state entities (QSEs). ‘Quasi-state entity’ is a novel concept introduced to distinguish actors in statehood conflicts that aspire to statehood, fulfil statehood functions to a greater or lesser degree, including, notably, the capacity and willingness to employ organised, restrained coercive violence, but which lack the status of sovereign statehood. QSEs overlap with, but are importantly and conceptually distinct from, nationalist movements, de facto states and rebels or insurgents. Legitimacy is a prerequisite for success, both for QSEs and for state entities. The legitimacy of an entity, its institutions and actions, in a certain constituency, at a certain moment, is difficult to ascertain, in its positive form. Legitimacy is best gauged by its actual or potential absence, at moments where an entity faces legitimacy crises, and where impact can be gauged through empirical observation of behaviour and in changing narratives and counter-narratives of legitimacy. International criminal procedures present direct legitimacy challenges for QSEs and (or) their adversaries. Legitimacy crises reveal both intended and unintended effects of international criminal justice on the legitimacy – and, so, the success, of QSEs.