Herein, a new ratiometric fluorescence probe is developed by the self-assembly of carbon nanodots and glutathione-stabilized gold nanoclusters (CNDs/GSH@Au NCs) to detect melamine. The red fluorescence of GSH@Au NCs can be quenched by Hg²⁺ and recovered by melamine, while the blue fluorescence of CNDs remains constant, so GSH@Au NCs serve as the signal section and the CNDs serve as the internal reference. This CNDs/GSH@Au NCs–Hg²⁺ system exhibits high sensitivity for melamine in the linear range from 0.1 to 30 μM with a detection limit of 29.3 nM. Moreover, this system also exhibits good selectivity, and some interfering substances, such as metal ions, amino acids, sugars and nitrogen-containing organic molecules, have no distinct effect on the determination of melamine. Benefiting from the high sensitivity and selectivity, the satisfactory recoveries are obtained in the detection of melamine in food samples, including milk, infant formula, and cat food.