We report a highly sensitive electrochemical and fluorescence sensor for the detection of toxic environmental pollutant Hg²⁺ ions by using a novel dextrin-cysteine-Schiff base (DCS). The sensor is developed by the periodate oxidation of dextrin followed by Schiff base formation with an amino acid, cysteine. The new characterized DCS is used as an efficient and selective sensor for the detection of Hg²⁺ ions in aqueous solution both by electrochemical and fluorescence quenching process. The detection limit (LOD) by the electrochemical method is 0.74 nM which is much lower than that by the fluorometric detection, where LOD value is found to be 40.93 μM. The Schiff base/glassy carbon electrode (DCS/GCE) shows an excellent selectivity towards Hg²⁺ ions compare to other environmentally relevant ions where almost nil current response is observed. The DCS/GCE also exhibits remarkable sensing of Hg²⁺ in real water samples such as river and tap water. The electrochemical sensing of Hg²⁺ by DCS is found to be superior to that of fluorescence sensing.