In recent years, Populist Radical Right Parties (PRRPs) have gained significant ground in many Western democracies. While most studies focus on rural areas, this paper examines the role of population growth in explaining support for Vox, Spain’s leading PRRP, in urban areas. Spain offers a unique case due to its rapid urbanisation and the substantial presence of Vox in metropolitan settings. Using data from over 27,000 urban census tracts, we find that population growth is positively associated with Vox support, particularly in suburban areas. This effect is moderated by socio-economic conditions: it is stronger in neighbourhoods with lower incomes and higher unemployment. Our findings suggest that population growth intensifies support for Vox in left-behind urban areas, underlining the interaction between demographic change and socio-economic disadvantage. These results expand the current understanding of PRRP support by showing that both rural depopulation and urban growth contribute to their success. However, the study’s limitations call for further research across different contexts and with more fine-grained indicators of left-behindness