Nitroxyl (HNO) is a derivative of nitric oxide (NO) that plays an essential role in various biological and pharmacological events. Until now, the in situ trapping and specific detection of HNO in living samples is still challenging. In this project, we fabricated a novel BODIPY-based micellar nanoprobe for monitoring nitroxyl in vitro and in vivo in ratiometric mode in aqueous solution. The probe (P-BODIPY-N) contains an asymmetrical BODIPY dye for fluorescent signaling and a diphenylphosphinobenzoyl as the trigger moiety; then we encapsulated P-BODIPY-N into the hydrophobic interior of an amphiphilic copolymer (mPEG-DSPE) and prepared a novel BODIPY-based micellar nanoprobe: NP-BODIPY-N. As far as we know, this probe is the first reported ratiometric fluorescent nanoprobe for HNO, which exhibits ultrasensitivity, high selectivity, and good biocompatibility. Above all, this nanoprobe shows favorable cellular uptaken and was successfully used to detect intracellular HNO released by Angeli's salt in living cells and zebrafish larvae. These results indicate that our newly designed nanoprobe will provide a promising tool for the studies of HNO in living system.