A water-soluble boronate-based fluorescent probe was evaluated for the detection of peroxynitrite (ONOO-) in the presence of a monosaccharide. The enhanced fluorescence of the probe when bound with d-fructose was switched off in the presence of peroxynitrite. In contrast, other reactive oxygen/nitrogen species (ROS/RNS) led to only slight fluorescence decreases due to protection by an internal N-B interaction. The interaction of the probe with d-fructose not only strengthens the fluorescence signal, but also protects the boronic acid from oxidation by other ROS/RNS. Therefore, under conditions generating various ROS/RNS, the boronate-based saccharide complex preferentially reacts with peroxynitrite. The probe was used in cell imaging experiments for the detection of endogenous and exogenous peroxynitrite. The sensor displays good "on-off" responses towards peroxynitrite, both in RAW 264.7 cells and HeLa cells.