A novel reversible fluorescence turn-on chemosensor 1 for Hg²⁺ was constructed based on a new receptor composed of an S atom and an alkene moiety. The sensor showed a large fluorescence enhancement response (1000-fold enhancement), high selectivity, and high sensitivity with a detection limit of 27.5 nM. Significantly, the reversible sensor functioned well in the near pure aqueous medium and could be employed for Hg²⁺ imaging in living cells.