Reactive oxygen species (ROS) and cellular oxidant stress have long been associated with cancer. Unfortunately, the role of HClO in tumor biology is much less clear than for other ROS. Herein, we report a BODIPY-based HClO probe (BCIO) with ultrasensitivity, fast response (within 1 s), and high selectivity, in which the pyrrole group at the meso position has an "enhanced PET" effect on the BODIPY fluorophore. The detection limit is as low as 0.56 nM, which is the highest sensitivity achieved to date. BCIO can be facilely synthesized by a Michael addition reaction of acryloyl chloride with 2,4-dimethylpyrrole and applied to image the basal HClO in cancer cells for the first time and the time-dependent HClO generation in MCF-7 cells stimulated by elesclomol, an effective experimental ROS-generating anticancer agent.