<p>Urbanisation and the associated low availability of green spaces in the cities affect the activity and diversity of insectivorous bat species worldwide. However, increasing the extent of green cover within large urban areas usually benefits bat communities and enhances the ecosystem services they provide, such as suppressing insect populations. Here, we studied the bat species that occur in urban green spaces of Madrid, a large European metropolis, determining the minimum area of urban vegetation that maximises bat richness and foraging activity. Bats were sampled using low-cost acoustic passive recorders in an extensive network of 77 sampling points. Our results showed a positive and significant relationship between the extent of urban green spaces and both bat richness and activity, with a minimal area between 7 and 9 hectares being necessary to maintain them. In addition, we found that insectivorous bat assemblages are more homogenous within reduced urban green areas due to species loss. These findings have practical implications for urban planning and management, as they can inform the development of more efficient approaches to preserving urban fauna diversity. Our results also highlight the importance of protecting and increasing green cover in urban settlements, which can also help improve the general quality of life and health of their citizens.</p>