A rapid and sensitive colorimetric assay was developed using cysteamine modified gold nanoparticles for the direct detection of nitrates in water samples. Gold nanoparticles stabilized with citrate were modified with cysteamine that has excellent affinity for nitrates, and its capacity to capture nitrates was evaluated, in comparison with other anions. The presence of nitrates in water samples could be tracked by naked eye with a color variation of the colloidal suspension from red to gray, and these results were confirmed through ultraviolet–visible measurements within a nitrate concentration of 35 ppm. In field analysis was performed in underground water extracted from wells during the year 2012 in Arborea area (Italy), a nitrate vulnerable zone, and information of nitrate concentration in the range of the recommended nitrate level in water was studied. This simple assay can be used for onsite detection of nitrates in water without the need for skilled personnel, sample pretreatment or expensive instrumentation.