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Cyanide (CN−) is one of the most hazardous ions to humans and the environment. Therefore, sensitive, and selective sensors for CN− monitoring are highly required. A novel chemosensor based on naphthalene flanked diketopyrrolopyrrole dye (TBC-DPPN) is synthesized for the direct detection of CN− in water samples by naked eye and vibrational spectroscopy (UV–Vis and fluorescence). The electron deficient carbonyl group of the lactam ring of TBC-DPPN undergoes a nucleophilic attack by the CN− thus causing the yellow color of the dye turn to colorless, and fluorescence of the dye at 527 nm to turn-off. The mechanism of CN− sensing by the TBC-DPPN sensor is confirmed by spectral measurements and density functional theory (DFT) calculations. The TBC-DPPN sensor is utilized to determine the CN− by colorimetric and fluorescence methods in water down to 0.5 and 0.05 µm, respectively, which is well below the cut off limit of 1.9 µm that is recommended by the World Health Organization (WHO). Therefore, TBC-DPPN can act as dual channel sensor with high selectivity and sensitivity for the determination of CN− in water.