As a sensitive and selective analytical technique, gold nanoparticles-based colorimetric sensing was characterized by its simplicity and cost-effectiveness. Specific methods have been extensively developed for different targets in diverse samples. In this study, a label-free method for sensing Co 2+ in aqueous solutions was described. The target was achieved by the induced aggregation of thiosulfate (S 2 O 3 2À ) stabilized gold nanoparticles (AuNPs) in the presence of ethylenediamine (en). Co 2+ first reacted with en and formed complexes of Co(en) 3 2+ in aqueous solutions, which was followed by the oxidation of Co(en) 3 2+ to Co(en) 3 3+ by dissolved oxygen. Co(en) 3 3+ then attacked S 2 O 3 2À ligands adsorbed on the AuNPs' surfaces, forming positively charged (en) 2 CoS 2 O 3 + on the AuNPs' surfaces, which reduced the surface charges of AuNPs and induced the aggregation of AuNPs. The process was accompanied by a red-shift in the adsorption spectrum and a visible colour change from wine red to blue. Potential effects of relevant experimental conditions, including pH, concentrations of S 2 O 3 2À and en, and incubation time were evaluated for optimization of the method. The proposed method is sensitive (LOD ¼ 0.0 4 mM or 2.36 ppb) and selective (by at least 100-fold over other metal ions except for Cu 2+ ) toward Co 2+ with a linear range from 0.1 to 0.7 mM. The cost-effective method allows rapid and simple determination of the concentrations of Co 2+ ions in drinking water.