Copper is a highly toxic environmental pollutant with bio-accumulative properties. Therefore, it is highly desirable to develop a simple, sensitive, and selective assay for Cu²⁺ recognition. Herein, a colorimetric sensor for Cu²⁺ with high selectivity and sensitivity using gold nanoparticles (AuNPs) is presented. AuNPs were first stabilized by polyvinylpyrrolidone (PVP). 2-Mercaptobenzimidazole (MBI) would cause the aggregation of PVP stabilized AuNPs (PVP-AuNPs) for mercapto ligands self-assembled on the surface of AuNPs. This would lead to a color change from red to purple. In contrast, with the addition of Cu²⁺, MBI would be responsive exclusively towards Cu²⁺. This would block mercapto ligand assembly and AuNPs remained monodispersed, exhibiting a red color. Therefore, taking advantage of this mechanism, a “purple-to-red” colorimetric sensing strategy could be established for Cu²⁺ detection. With this strategy, the concentration of Cu²⁺ can be detected with the naked eye or with ultraviolet-visible spectroscopy, and the detection limits of Cu²⁺ were 5 μM and 0.5 μM, respectively. Additionally, the proposed method shows excellent anti-interference capability against many other metal ions, and real water sample applicability. Taken together, these advantages make this assay simple and robust and therefore promising for on-site water testing.