Display omitted •The rationally designed receptor forms Hg(II) complex easily.•The Hg(II) complex senses I− via turn-on fluorescent in aqueous solution.•The mechanism of sensing involves displacement of metal ion from the complex.•This complex exhibited many advantages than similar in-situ generated Hg(II) complexes. A simple Schiff base ligand (L) bearing 3,5-dibromo-2-hyrdoxybenzaldehyde fluorophore moiety has rationally been designed, synthesized and characterized. The Hg(II) complex of L has been synthesized by simply stirring a mixture of L and HgCl2 in ethanol at RT and the resultant non-fluorescent complex HgL is found to detect iodide ion selectively, with a turn-on fluorescence in H2O:DMSO (1:1 v/v) medium. The detection of iodide is performed via the metal ion displacement mechanism with a 1:2 (Probe:Iodide) stoichiometry and binding constant of 1.51x104 M−1. The results indicated that novel HgL complex showed rapid response towards iodide ion with a limit of detection (LOD) as low as 60 nM. Moreover, the sensing mechanism of iodide by the complex is theoretically supported by density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculations. In addition, estimation of iodide in real water samples, table salt and urine sample demonstrate the practical value of the probe.