In this study, we describe the anion recognition ability of isatin-3-thiosemicarbazone 2, which contains two different anion recognition units i.e. isatin NH and the thiourea moiety. Both have the ability to act as proton donors. Most importantly, a significant colour change of 2 was observed (from light yellow to reddish orange) in organic medium only after the addition of the F^– anion. No such colour change could be observed for any other anions including Cl^–, Br^–, I^–, H₂PO₄⁻,NO₂⁻,PF₄⁻etc. The UV-Vis spectroscopic studies also indicate the potential of this compound for selective detection of fluoride anions. ¹H-NMR titrations clearly indicate the formation of the 2.F^– anionic complex. The Density-functional theory (DFT) calculations are also performed to get further insights on the formation of 2.F^– complex.