<p>Water hardness plays a critical role in regulating pollutant toxicity to freshwater aquatic organisms. It can significantly influence the bioavailability and toxicity intensity of pollutants. Water hardness varies regionally and optimal levels for freshwater aquatic organisms are often species specific. This study employed a meta-analysis approach to systematically evaluate the regulatory effects of water hardness on pollutant toxicity to freshwater aquatic organisms, supplemented by qualitative analysis to provide additional evidence. We systematically searched the Web of Science database to screen and integrate data from 32 eligible studies. Standardized mean differences (SMD) were calculated using random-effects models, with heterogeneity assessed through I² statistics. Qualitative analysis revealed that 20 out of the 23 studies demonstrated significant inverse correlations between water hardness and pollutant toxicity. High-hardness water reduces the bioavailability of pollutants through competitive mechanisms involving calcium and magnesium ions, which mitigates the toxic effects on freshwater aquatic organisms. Quantitative analysis demonstrated that high water hardness exerted a statistically significant protective effect against pollutant toxicity. However, significant heterogeneity was observed among studies, with developmental stages of species, chemical forms of pollutants, and differences in hardness gradient design identified as major confounding factors. This study integrated the effect relationship between water hardness and pollutant toxicity in multiple regions around the world by meta-analysis, confirming that hardness is a key moderating factor for pollutant toxicity.</p>