A simple label-free colorimetric sensor for sulfate has been devised based on the analyte-induced shielding of peroxidase-like activity of cysteamine-capped gold nanoparticles (cyst-AuNPs). Aggregation triggered by sulfate through hydrogen bond and electrostatic interaction gave rise to a remarkable decrease of the catalytic activity of AuNPs. By coupling with 3,3′,5,5′-tetramethylbenzidine (TMB)–H2O2 chromogenic reaction, a novel colorimetric sensor for detection of sulfate was established. Based on the signal amplification effect of the catalytic reaction, a detection limit as low as 0.16μM was obtained by virtue of UV–vis spectroscopy or 4μM with the naked-eye observation. The as-proposed sensor exhibited an excellent selectivity over other common anions and metallic ions. In view of these characteristics, such sensing probe features simplicity, cost-effectiveness, and naked-eye screening. These advantages make this sensor a powerful protocol for the quantitative detection of sulfate in water samples with satisfactory results.