A simple and sensitive colorimetric sensing method was constructed for detection of Hg2+ in aqueous solutions and based on silver nanoparticles functionalized with l‐cysteine (l‐Cys–Ag NPs). In this method, adenosine triphosphate (ATP) induced aggregation of l‐Cys–Ag NPs. Simultaneously, the solution colour changed from bright yellow to brown. In the presence of Hg2+, Hg2+ chelated ATP to form a complex and reduce the degree of aggregation of l‐Cys–Ag NPs and was accompanied by a colour change from brown to bright yellow. The changing values of absorbance at 390 nm were linearly correlated with concentration of Hg2+ over the 4.00 × 10−8 to 1.04 × 10−6 mol·L−1 range, with a detection limit of 8 nM. This method was used successfully for detection of Hg2+ in real water samples and performed good selectivity and sensitivity. The recovery range was 91.5–109.1%, indicating that the method has vast application potential for determination of Hg2+ in the environment.