The deficiency, as well as overconsumption of essential metal ions, has serious health implications. Hence, the design and synthesis of sensors for detection of such ionic species are of great significance. In this work, a Dehydroacetic acid-based chemosensor DHB ((E)-N-(1-(2-hydroxy-6-methyl-4-oxo-4H-pyran-3-yl) ethylidene benzohydrazide) was developed for instant detection of Cu2+. A visible colour transition from light yellow to bluish green was observed on the addition of Cu2+. No transition in colour was observed with any other ionic species indicating its specificity. The binding of DHB to Cu2+ caused a blue shift accompanied by an increased intensity due to an intramolecular charge transfer (ICT). The absence of any interference from other competitive ionic species confirmed the selectivity of the probe for Cu2+. Single-crystal X-ray analysis, titration studies and Job's plot, indicated a 1:2 stoichiometry between Cu2+ and DHB. The experimental results agreed with the theoretical results obtained from Density Functional Theory (DFT) calculations. The successful application of the probe for the detection of Cu2+ in spiked real samples of water collected from different sources affirmed its practical applicability. The sensor could detect Cu2+ as low as 0.156 μM in the aqueous media which is very less than the permissible value of 20 μM in water.