A novel coumarin-based "turn-on" fluorescent probe SS-1 was developed for selective detection of SO2 derivatives in 100% water solution among reactive sulfur species (RSS). The detection limit of the method was 91.7 nM. Moreover, the probe exhibited low cytotoxicity and was successfully applied in the imaging of endogenously generated SO2 in HeLa cells. endogenously generated SO2, fluorescent probe, bioimaging Citation: XQ. A novel coumarin-based water-soluble fluorescent probe for endogenously generated SO2 in living cells. Sci China Chem,