In this paper, a fluorescein spirolactam derivative, ((E)-3′,6′-dihydroxy-2-((thiazol-2-ylmethylene)aminospiro[isoindoline-1,9′-xanthen]-3-one) (HTMIX), was synthesized and used as a turn-on fluorescence probe for the detection of silver ion in aqueous solution. The binding mechanism of HTMIX to Ag⁺ was evaluated using the Hildebrand–Benesi equation based on a 1 : 1 binding model with R = 0.9993. And the influence of sixteen common metal ions on the fluorescence intensities of HTMIX–Ag⁺ solution was investigated in detail. The results show that HTMIX exhibited high selectivity towards Ag⁺ in a 20% ethanol solution. The obtained fluorescence probe was used for the quantitative determination of Ag⁺ with a good linear range from 0.1 μM to 10 μM (R = 0.9969) and a satisfactory detection limit of 0.08 μM. In addition, the present probe has been further used for the detection of Ag⁺ in tap water, river water and lake water. And the accuracy of the results obtained by the proposed method was in good agreement with that obtained by flame atomic absorption spectrometry.