This article reviews the literature on the causes of welfare state expansion in democratic middle‐income countries across the Global South since the 2000s. After discussing challenges to measuring welfare state change, the article reviews research in political science and sociology and discusses nine potential causes of recent welfare state expansion, namely (a) economic development, (b) fiscal capacity, (c) democracy, (d) partisan ideology, (e) labour unions, (f) social mobilization, (g) cultural homogeneity, (h) institutional architecture, as well as (i) welfare rights and norms. The review thus reveals that recent welfare state expansion in democratic middle‐income countries has been driven by a similar set of causes as post‐war welfare state expansion in the Global North. The pronounced expansion of non‐contributory social policies has, in some contrast, also been characterized by more bipartisan and transnational political dynamics. The article concludes by outlining avenues for future research and by calling for more scholarly attention to the consolidation and testing of existing theories.