Hypochlorous acid (HClO) is a significant signaling species which plays a decisive role in the immune system as an excellent defense against invasive bacteria. Therefore, it is significant to monitor HClO accurately and quickly in organisms. Herein, a new turn-on fluorescent probe (BM-HA) with a simple structure for rapid sensing of HClO is presented. BM-HA has favorable properties, including high selectivity, a large turn-on fluorescence signal (140-fold), and fast response (about 15 s). Additionally, quantum chemical calculations with DFT and TDDFT proved that reduced fluorescence of the BM-HA probe was possible due to the a-PET effect and the non-coplanarity of the coumarin and the benzothiazole. Furthermore, imaging in living cells and zebrafish indicates that the probe can potentially be used to detect HClO in complex biosystems.