PurposeThis article examines the links between average city size, fiscal decentralisation, and national economic growth in 33 Organisation for Economic Co-operation and Development (OECD) countries.Design/methodology/approachThe data in this paper comprise an unbalanced panel dataset which contains economic growth indicators, average city size, fiscal decentralisation indicators and control variables in 33 OECD member countries from 1975 to 2015 in five-year intervals. Fixed-effects (FE) estimators are used for the analysis.FindingsThis research finds i) countries with larger weighted average city sizes have higher economic growth, ii) countries with greater fiscal decentralisation have higher economic growth, but iii) countries with larger weighted average city sizes with greater decentralisation have lower rates of economic growth.Originality/valueThe research highlights the importance of agglomerations and decentralised governance and management for economic growth. While the findings are consistent with previous evidence that larger city sizes and fiscal decentralisation are separately associated with higher rates of economic growth, the authors find countries which have larger cities and greater fiscal decentralisation experience lower rates of economic growth highlighting a need for caution on decentralisation agendas in such cases. The implications of this suggest policymakers should proceed with caution on decentralisation agendas in countries with large cities.