We demonstrate for the first time a highly selective anion-responsive reversible gel–sol state transition in a supramolecular hydrogel of Ag(I)-glutathione (GSH) coordination polymers, which allows for facile and selective visual recognition of I−via naked eyes even in a strongly colored and/or fluorescent background. Such a strategy overcomes the drawback of spectral interferences which are often encountered in conventional colorimetric and fluorimetric means. It was rationalized that I− functioned as a depolymerizing agent for the Ag(I)-GSH supramolecular hydrogels. A feasible quantitative assay for I− was established that afforded satisfactory results for simulated wastewater samples. We believe that this strategy can in principle be applicable to other species by following a smart gel–sol state transition in designed supramolecular hydrogel.