Urban streams are increasingly shaped by interactions between biophysical processes and uneven social infrastructure, yet socioeconomic drivers of water quality degradation remain underrepresented in freshwater science. We evaluated physicochemical and microbiological water quality in urban streams of the Billings–Tamanduateí sub-basin, São Paulo, Brazil, and examined their relationships with social and sanitation indicators.Water samples were collected across 13 urban sub-basins and analyzed for dissolved carbon and nitrogen fractions, basic physicochemical parameters (pH, dissolved oxygen, conductivity, turbidity), and fecal indicator bacteria (Escherichia coli, Enterococcus spp., and Pseudomonas aeruginosa). These environmental data were integrated with socioeconomic variables including population density, per capita income, informal settlements, and access to sewerage.Multivariate and correlation analyses revealed clear gradients linking degraded water quality to higher population density, limited sanitation coverage, and increased social vulnerability. Sub-basins characterized by lower income and greater prevalence of informal settlements exhibited elevated organic matter concentrations and higher occurrence of fecal indicator bacteria, indicating intensified anthropogenic pressures on urban freshwater systems.Our results indicate that social inequality is a central driver of ecological degradation in urban streams and underscore the importance of incorporating social indicators into urban freshwater monitoring and management.