A highly selective fluorescent probe (BON) for hypochlorite (ClO−) was designed and synthesized. The probe was based on boron-dipyrromethene (BODIPY) dye as the fluorophore and diaminomaleonitrile (DAMN) as the recognition group. Due to the isomerization of C=N, BON has almost no fluorescence and produces bright green fluorescence after recognition of ClO−. The carbon–nitrogen double bond (C=N) connecting DAMN with BODIPY may be oxidized by hypochlorous acid to an aldehyde group. The combined action of the nitro and the aldehyde groups has created a double attraction (A–π–A), which increases the fluorescence intensity by 10 times. BON displayed excellent properties such as high selectivity, an excellent response time (5 min), a low detection limit (LoD = 0.27 μM) and excellent stability. Moreover, BON has been applied for the detection of ClO− in tap water with spiked recoveries in the range of 90.5–93.6%, and BON has been shown to be suitable for the detection of exogenous hypochlorous acid in living cells as well. This means that the probe BON might be a new efficient tool in detecting the roles of hypochlorous acid. [ABSTRACT FROM AUTHOR]