Abstract A new rhodamine B‐based fluorescent probe for the hypochlorite anion (OCl − ) has been designed, synthesized, and characterized. The probe comprises a spectroscopic unit of rhodamine B and an OCl − ‐specific reactive moiety of dibenzoylhydrazine. The probe itself is nearly nonfluorescent because of its spirolactam structure. Upon reaction with OCl − , however, a largely enhanced fluorescence is produced due to the opening of the spirolactam ring by the oxidation of the exocyclic hydrazide and subsequently the formation of the hydrolytic product rhodamine B. Most notably, the fluorescence‐on reaction shows high sensitivity and extremely high selectivity for OCl − over other common ions and oxidants, which makes it possible for OCl − to be detected directly in their presence. In addition, the reaction mechanism has been investigated and proposed. The OCl − anion selectively oxidizes the hydrazo group in the probe, and forms the analogue of dibenzoyl diimide, which in turn hydrolyzes and releases the fluorophore. The reaction mechanism that is described here might be useful in developing excellent spectroscopic probes with cleavable active bonds for other species.