The detection of the lead heavy metal (Pb) in water is crucial in many chemical processes, as it is associated with serious health hazards. Here, we report the selective and precise colorimetric detection of Pb ions in water, exploiting the aggregation and self-assembly mechanisms of glutathione (GSH)-functionalized gold nanoparticles (GNPs). The carboxyl functional groups are able to create coordination complexes with Pb, inducing aggregation amongst the GSH-GNPs in the presence of Pb due to the chelation of the GSH ligands. The resulting aggregation amongst the GSH-GNPs in the presence of Pb increases the aggregate size depending on the available Pb ions, affecting the plasmonic coupling. This causes a substantial shift in the plasmon wavelength to a longer wavelength side with increasing Pb concentration, resulting in a red-to-blue colorimetric or visual change, enabling the instant determination of lead content in water.