A FRET-based fluorescent probe for the detection of sulfide and/or sulfite in aqueous buffer, was constructed by connecting the quencher moiety nitrobenzofurazan (7-nitro-1,2,3-benzoxadiazole, NBD) to a water-soluble 3-(2-benzimidazolyl)-7-hydroxycoumarin carboxylic acid through a piperazine linker. This probe exhibits good selectivity and high sensitivity for sulfite and sulfide over cysteine and other potential analytes. Furthermore, this is one of the few examples of fluorogenic coumarins whose solubility in water is maintained upon its conversion into reaction-based fluorescent probes.