Scholarly interest in radical Islam is long-standing and crosses multiple disciplines. Yet, while the labelling of Islam and Muslim actors as ‘radical’ is extensive, this has not been interrogated as a particular scholarly practice. And while studies of non-Western radicalism have grown in recent years, cross-cultural analysis of radicalism as a particular concept in political thought has been neglected. This article aims to begin to address this question, with reference to radical Islam. By treating radicalism as a meta-concept, it identifies radical Islam as a malleable and composite category that is constituted by, and made legible through, conceptual properties associated with four discourses in the study of radicalism with origins in the Western academy: Euro-radicalism, identified with the European left and critical theory; fundamentalism; radicalisation; and liberalism. I argue that radical Islam is under-theorised and over-determined as a scholarly category. This can be explained by how concepts originating in the Western academy to address Western contexts and phenomena function as master frameworks, narratives, or pivots against or around which radical Islam is defined. This is the case even when Eurocentrism is contested by critical theorists who tend to reproduce it because they do not abandon Western conceptions of radicalism but rather draw on them. Academic accounts of radical Islam also authenticate Islam by advancing selective, strategic or apologetic descriptions of what constitutes radicalism. In these ways, critical scholarship, including within IR, can also be insufficiently attentive to marginal and heterodox voices that fall outside hegemonic conceptions of Islamic normativity.