A sensitive and selective cysteine colorimeric sensor based on the interaction between cysteine, silver nanoparticles (AgNPs) and calcium ions has been developed. In the presence of Ca2+ and NaCl, cysteine could rapidly induce the aggregation of AgNPs, thereby resulting in a yellow-to-red color change. The presence of 10 mM NaCl in the samples decreases the electrostatic repulsion and accelerates the aggregation of AgNPs. Results demonstrated that other amino acids cannot change the color of AgNPs solution in the same conditions, probably due to the absence of the thiol groups, suggesting the selectivity of the proposed method toward cysteine. The color change was monitored by naked eye and UV-Vis spectrophotometry. The ratio of absorption at 524 to 396 nm (A(524)/A(396)) is linearly dependent on the cysteine concentration in the range of 0.25-10 mu M.