Urbanization strongly impacts biodiversity and related ecosystem services, making its proper evaluation essential, with a specific focus on taxonomic groups providing fundamental ecosystem services. Among these, spiders are renowned regulators of trophic chains and sensitive bioindicators. We assessed the response of foliage-dwelling spiders to urbanization in Turin (Italy), by sampling their communities in urban green areas along an urbanization gradient and in a natural park (control area) a few kilometers from the city. We built the urbanization gradient by calculating and comparing six landscape fragmentation metrics (effective mesh size, landscape division, splitting index, coherence, splitting density, net product) plus building density within 1000 m buffers around each sampling point. Among these, building density was the best-performing proxy, as it was strongly correlated with all other fragmentation metrics and most effectively explained the observed patterns in species diversity. Species richness, abundance, and functional diversity declined along the urbanization gradient, with the lowest values observed in the city. This trend, largely driven by the loss of specialized foraging guilds (e.g., pollinator-feeders), mirrors previous findings on ground-dwelling spiders. Species turnover explained most of the variation among urban green sites, reflecting stochastic dynamics, while urban communities appeared as subsets of semi-natural ones, indicating environmental filtering favoring urban-adapted species. We also assessed current taxonomic and functional diversity responses and projected biodiversity trends under future scenarios aligned with the EU restoration law, which calls for a 3% increase in urban green areas by 2040 and 5% by 2050. As all diversity metrics were strongly correlated, we synthesized them into a single biodiversity index. Simulations suggest that even modest increases in green space can significantly enhance urban biodiversity. Overall, our results highlight how urbanization reshapes spider communities, favoring generalists and selected traits.