Hypochlorous acid (HOCl) is widely recognized as a reactive oxygen species of considerable importance in biological systems, and its abnormal metabolism is closely related to various diseases. Therefore, accurately tracking and detecting the concentration levels of HOCl in living organisms hold significant importance in exploring its biological functionalities and related pathological mechanisms. In this work, a coumarin-based fluorescent probe, Cse, containing a selenide group has been developed for fluorescence imaging intracellular and zebrafish HOCl levels. On the basis of the specific oxidation reaction between hypochlorous acid and phenyl selenide, Cse displayed a remarkable fluorescence turn-on response to hypochlorous acid (over 30-fold increase) with a notable Stokes shift (approximately 112 nm), rapid response kinetics (K_obs = 0.389 min⁻¹), and a low detection limit (10.5 nM). Moreover, biological imaging experiments demonstrated that probe Cse enables the visualization of HOCl in real-time through fluorescence imaging techniques, both in living HeLa cells and zebrafish specimens. The successful advancement of this probe provides a novel fluorescence imaging tool to investigate the biological mechanisms associated with diseases related to HOCl.