Hypochlorous acid (HClO), one of the reactive oxygen species (ROS), is a key microbicidal agent which is used for natural defense; however, it is also involved in several human diseases. Although several efforts have been made to develop HClO-selective fluorescent sensors, most of them display delayed response time and the ratiometric sensors are rare. In the current work, we used a 3-pyrrole BODIPY dye as a new ratiometric fluorescent sensor (BRClO) for HClO. BRClO exhibited a 177-fold fluorescence ratio (F-505 nm/F-585 nm) enhancement in the presence of HClO over other ROS with fast response time (completed within 1 s). Confocal fluorescence imaging with BRClO showed separated signals in the presence of exogenous HClO in live cells. In addition, BRClO could be easily prepared as HClO test strips to detect HClO in aqueous solution and natural tap water.