The recently emerging gold nanoclusters (GNC) are of major importance for both basic science studies and practical applications. Based on its surface-induced fluorescence properties, we investigated the potential use of Au25(SG)18 (GSH: glutathione) as a fluorescent iodide sensor. The current detection limit of 400 nM, which can possibly be further enhanced by optimizing the conditions, and excellent selectivity among 12 types of anion (F−, Cl−, Br−, I−, NO3−, ClO4−, HCO3−, IO3−, SO42−, SO32−, CH3COO− and C6H5O73−) make Au25(SG)18 a good candidate for iodide sensing. Furthermore, our work has revealed the particular sensing mechanism, which was found to be affinity-induced ratiometric and enhanced fluorescence (abbreviated to AIREF), which has rarely been reported previously and may provide an alternative strategy for devising nanoparticle-based sensors.