A simple, rapid colorimetric method using label-free Ag NPs (silver nanoparticles) as probe for detection of melamine in raw milk was developed in this study. The assay relies on the fact that melamine can induce aggregation of Ag NPs, and thereby results in their yellow-to-red color change. The concentration of melamine in raw milk can be determined by monitoring with the naked eyes or a UV–Vis spectrophotometer. The detection limit of the present method for melamine is 2.32 μM (3σ). The proposed method is a promising mean for on-site screening of melamine adulterant in raw milk without costly instruments.