A carbazole–thiosemicarbazone–Hg²⁺ ensemble-based fluorogenic probe for detection of iodide in aqueous media is reported. The first fluorescent sensor for iodide anions was constructed based on the displacement approach. An ‘ensemble’ is able to selectively sense iodide over other anions followed by the release of 9-(butane-1-yl)-9H-carbazole-3,6-dihydrazinecarbothioamide to give a remarkable change of fluorescence turn-on signal at pH 7.4 under aqueous media. The practical use of an ‘ensemble’ was demonstrated by its application to the detection of iodide in the living cells.