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 that of other ROS. Herein, we report a BODIPY-based HClO probe (BClO) with ultrasensitivity, fast response (within 1s), and high selectivity, in which the pyrrole group at 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. BClO 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.