In this study, we have utilized a simple and sensitive colorimetric assay using unmodified gold nanoparticles to detect sulfanilamide in the presence of dopamine. The remarkable point of our analytical assay is based on the color change of the gold nanoparticles solution according to their dispersion or agglomeration state. Furthermore, the presence of sulfanilamide in the dopamine-coated nanogold solutions retarded the aggregation of the nanoparticles, leading to a delay in the color change. The impacts of pH, the initial concentrations of dopamine and sulfanilamide, and different interferents on the colorimetric response were investigated. The limit of detection (LOD) is 0.3 μM, which is lower than those of previously reported methods for sulfanilamide determination under the optimal conditions (0.1 mM dopamine concentrations, pH 7.0, incubation time of 15 minutes). For quantitative measurement using UV-Vis spectra, a good linear relationship ( R 2 = 0.9920 and R 2 = 0.9951) between the sulfanilamide concentration and absorption intensity was obtained in the ranges of 0.5-20 and 20-80 μM, respectively. The proposed method also showed low interference with the co-existence of some amino acids and organic chemicals in the mixture. Moreover, it was also applied to determine sulfanilamide in milk and pork samples, with good quantitative recoveries varying in the range of 97.2-101.9% and 98.3-107.7%, respectively. Increasing the sulfanilamide concentration changed the color of the dopamine-coated gold nanoparticles gradually from blue to red, which indicates that our proposed assay unlocks great potential in sulfanilamide analysis with naked-eye readout. In this study, we have utilized a simple and sensitive colorimetric assay using unmodified gold nanoparticles to detect sulfanilamide in the presence of dopamine.