A simple and sensitive method for colorimetric detection of mercury ion (Hg(2+)) has been developed by using a poly (γ-glutamic acid) functionalized gold nanoparticles (PGA-AuNPs) system. Electrostatic self-assembly technique was used to assemble negatively charged PGA on the surface of positively charged CTAB-capped AuNPs. With the increase of Hg(2+) concentration, the color of the solution would progress from light red to purple blue. The results showed that the absorbance ratio (A750/A580) was linear with the Hg(2+) concentration in the range of 0.01-10 μM and from 50 to 300 μM, with the correlation coefficients of 0.998 and 0.991, respectively. The reported probe is suitable for real-time detection of Hg(2+) in water with the limit of detection (LOD) of 1.9 nM obtained by UV-vis spectrum, and exhibits selectivity toward one order of magnitude over other metal ions. This approach was applied successfully to the determination of Hg(2+) in tap water and mineral water, and the recoveries were from 90% to 103% and from 103.53% to 113%, respectively. The proposed method is rapid, low-cost and free of complex equipment, making it possible to analyze Hg(2+) in various water samples.