Gold nanoparticles (AuNPs) were synthesized using L-histidine as a reducing and capping agent for the detection of environmental contaminant Hg2+ in water. Synthesis was done by combining HAuCl4 solution with L-histidine at pH 11 to produce red AuNPs(his) solution. The zeta potential measurement showed AuNPs(his) had a negative surface charge of −22.4 mV. Characterization using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) showed the formation of AuNPs(his). Morphology analysis of AuNPs(his) shows a spherical shape. XRD measurement showed that the addition of Hg2+ caused an increase in the crystal size of Au0 in the (111) plane from 13.96 to 34.34 nm, which was calculated using the Scherrer equation. Characterization using a particle size analyzer (PSA) showed the formation of peaks at wavelengths around 525 nm, with an average particle size of 17.9 nm. The addition of Hg2+ ions affects the aggregation of nanoparticles, causing a color change from red to purple that can be seen visually. The decrease in stability is characterized by a redshift to 560 nm. Therefore, AuNPs(his) can be developed as Hg sensors with the advantages of fast response, ease of use, and the ability to detect color changes with the naked eye.