A new excited-state intramolecular proton transfer (ESIPT) based fluorescent chemodosimeter for the detection of Hg²⁺ has been rationally designed and developed. The probe operates via the specific mercury-promoted desulfurization reaction of thiophthalimide to phthalimide and exhibits high selectivity and sensitivity in an almost pure aqueous solution (containing only 1% DMSO) with a low detection limit of 1.5 ppb. Furthermore, the probe was successfully used for the fluorescence imaging of Hg²⁺ in live cells.