Although the rise of localism in Hong Kong has generated a large body of literature, an overarching definition of this new political force is still lacking. Based on analyses of organisations' documents, media reports and interviews with activists, this article argues that localist groups can be defined as groups that share a combination of three core ideological features: regionalism, radicalism and populism, and an origin in centre–peripheral conflicts between mainland China and Hong Kong. Given this definition, localism can be conceptualised as a kind of "populist radical regionalism". The implications of this definition will be discussed in this article.