Dansyl-labeled methionine is synthesized by solid-phase synthesis, and found to be a highly sensitive and selective sensor for Hg²⁺. The sensor sensitively detects Hg²⁺ ions in aqueous solution by a turn-on response; however, the sensor detects Hg²⁺ ions by a turn-off response in organic and mixed aqueous–organic solutions. We investigated the binding stoichiometry, binding constant, and binding mode of the sensor under various solvent conditions. In 100% aqueous solution, 2 : 1 complexation of the sensor with Hg²⁺ ions is more favorable than 1 : 1 complexation, whereas the sensor preferentially forms a 1 : 1 complex in 100% CH₃CN or in 50% CH₃CN–aqueous solutions. Results reveal that the stoichiometry of the sensor–Hg²⁺ complex plays an important role in the type of response to Hg²⁺ ions, and that 2 : 1 complexation is required for a turn-on response to Hg²⁺ ions in aqueous solution.