A new fluorescein-based probe is found to be sensitive and selective for the detection of hypochlorite anion in aqueous buffer solution. The probe displays high water solubility, pH-independent fluorescence, and biocompatibility. A possible detection mechanism is that HOCl oxidizes the CN bond into aldehyde, resulting in bright emission. Results show that the probe has a 7.3 nM detection limit to hypochlorite anion. In addition, the probe is successfully utilized for monitoring endogenously produced hypochlorite in living macrophage cells upon stimulation.