A novel water soluble Hg²⁺-selective chemosensor 1 with hemicyanine as fluorescent reporting group and NO₂Se₂ chelating unit as ion binding site was reported. Chemosensor 1 shows a specific Hg²⁺ selectivity and discrimination between Hg²⁺ and chemically similar ions in conjunction with a visible colorimetric change from red to colorless, potentially leading to both “naked-eye” and fluorometric detection of Hg²⁺ cations.