In this study, we demonstrated a convenient colorimetric method for the determination of SCN− by employing citrate-capped AuNPs as probes without further modification. We found that the citrate-capped AuNPs aggregated readily in the presence of sulfuric acid (8 mM), leading to a color change of the colloidal solution from red to blue. This particle aggregation was, however, selectively suppressed by SCN−, which had strong protective effect on the gold colloid. We applied this method to determine SCN− in real water samples, and satisfactory results were achieved.