<p>Bee populations are declining worldwide due to multiple stressors, such as urbanization. However, urban parks are regarded as potential biodiversity refuges, although the evidence remains contradictory. In this study, we used a biogeographic approach to evaluate the effectiveness of urban parks as refuges for bees and whether this varies across the early and late seasons. To do so, we characterized bee communities in urban parks and adjacent natural areas in a medium-sized city in the Iberian Peninsula during the whole period of bee activity. We assessed species richness, the nested structure of bee communities, and the contribution of nestedness versus turnover to beta diversity. Urban parks hosted lower bee richness than natural areas, but only in the late season. Despite this, bee communities in urban parks were not nested subsets of communities from natural areas, as more than 30% of species were idiosyncratic in both seasons. Such idiosyncratic species were rare in distribution and abundance, so that most of them were unique to specific urban parks or natural areas. Communities in urban parks strongly differed from those in natural areas, mostly due to species turnover. Thus, in our study system, urban parks functioned as biodiversity refuges throughout the year, hosting distinct communities with many unique species. Improving bee diversity and conservation in cities may rely on enhancing species turnover by establishing a network of scattered urban parks and on providing effective flowering and nesting resources for bees, especially to prevent the local extinction of idiosyncratic species.</p>