Hydrogen sulfide (H2S) is emerging as an important mediator of human physiology and pathology but remains difficult to study, in large part because of the lack of methods for selective monitoring of this small signaling molecule in live biological specimens. We now report a pair of new reaction-based fluorescent probes for selective imaging of H2S in living cells that exploit the H2S-mediated reduction of azides to fluorescent amines. Sulfidefluor-1 (SF1) and Sulfidefluor-2 (SF2) respond to H2S by a turn-on fluorescence signal enhancement and display high selectivity for H2S over other biologically relevant reactive sulfur, oxygen, and nitrogen species. In addition, SF1 and SF2 can be used to detect H2S in both water and live cells, providing a potentially powerful approach for probing H2S chemistry in biological systems.