Abstract The leather industry is water-intensive and generates highly polluted wastewater. Brazil, with the world’s largest commercial cattle herd, is a leading leather producer. The state of Rio Grande do Sul holds a prominent position in national exports, with a high concentration of tanneries in the Sinos River Basin. Thus, this study aimed to assess tannery wastewater treatment practices in the basin, quantify pollution loads, and evaluate their impact on surface water quality. Data from the State Environmental Agency were collected, including treatment technologies, discharge volumes, guideline exceedances, and pollutant concentrations. Frequent violations involved thermotolerant coliforms, nitrogen, total chromium, chemical oxygen demand (COD), oils and greases, and suspended solids. Pollution loads ranged from 27.2 kg/year (total chromium) to 94.5 t/year (COD), indicating a contribution from tanneries to basin contamination.