Abstract Freshwater ecosystems, particularly rivers, are experiencing the most rapid biodiversity declines of any biome, driven by several interacting stressors operating across local to global scales. Despite growing research on these interactions, the lack of systematic quantification of individual stressor gradients limits our ability to disentangle their cumulative effects. Here we present a global synthesis of stressor–response relationships across five key riverine organism groups—prokaryotes, algae, macrophytes, invertebrates and fish. We screened 22,120 papers and extracted 276 studies with 1,332 stressor–response relationships. We used generalized linear mixed models and Bayesian meta-analyses to quantify the response to the seven most prevalent stressors. Consistently across taxa, biodiversity loss (taxon richness and evenness) reflected elevated salinity, oxygen depletion and fine sediment accumulation, while the association with nutrient enrichment and warming varied among groups. Predictive tools, including hypothetical outcome plots and partial dependence plots, revealed the interplay of stressors and predicted biodiversity response to stress increase. Our findings establish a quantitative baseline for a continuous global synthesis, refining predictions of anthropogenic stressor impacts, identifying key research gaps and informing conservation strategies for freshwater ecosystems.