Here we report a simple colorimetric method for the detection of adenosine triphosphate disodium (ATP) using unmodified gold nanoparticles (AuNPs) as an optical probe. In acidic media, the melamine can link the neighboring AuNPs and result in the formation of the gold nanoparticle aggregates. Simultaneously, the color of AuNPs solution changes from red to blue. However, upon the addition of ATP, the responsive disassembly of the gold nanoparticle aggregates occurs, due to the competitive adsorption between the ATP and the melamine molecules on the surface of the AuNPs. Consequently, the aggregated AuNPs are dispersed, accompanied by the naked-eye visible color change from blue to red. Thus, a reverse assay procedure was established to light up the colorimetric sensing of ATP on basis of the ATP-triggered responsive disassembly of the melamine-crosslinked gold nanoparticle aggregates, and the logarithm of absorbance ratio of A₆₆₀/A₅₁₈ was found to be proportional to the ATP concentration over the range of 0.6–2.4 × 10⁻⁷ mol L⁻¹ with a detection limit (3σ) of 8.0 nM.