The synthesis and photophysical properties of a rosamine-based fluorescent chemosensor, RosAg, for detecting Ag ion in an aqueous solution are described. This fluorescent sensor has a negligible quantum yield (<0.005) in the absence of Ag+, whereas a significant increase in fluorescence is observed upon complexation with Ag+ under physiological conditions. The crystal structure of the silver complex with the chelator moiety of RosAg reveals a trigonal-planar coordination geometry in which three S atoms occupy the metal center. Although a strong coordinative interaction of Ag-N is not observed in the crystal structure, the H-1 NMR experiments suggest that aniline nitrogen is likely to be associated with the Ag+ center in the solution state. This may inhibit the photoinduced electron transfer process and result in the enhancement of fluorescence.