Methionine sulfoxide reductases (Msrs) play essential roles in maintaining mitochondrial function and are recognized as potential therapeutic targets. However, current probes for Msrs fail to target mitochondria and exhibit a relatively slow response and limited sensitivity. Here we develop a novel turn-on fluorescence probe that facilitates imaging of mitochondrial Msrs in living cells. The probe is constructed by conjugating a methyl phenyl sulfoxide, a mimic Msrs substrate, to an electron-withdrawing hydrophobic cation, methylpyridinium. The probe of acceptor-acceptor structure is initially nonemissive. Msrs catalyzed reduction of sulfoxide to sulfide generated a fluorophore of distinct donor-acceptor structure. The probe is demonstrated to exhibit high sensitivity, fast response, and high selectivity toward MsrA in vitro. Furthermore, the probe is successfully introduced to detect and image Msrs in living cells with excellent mitochondrial-targeting capability. Moreover, the probe also reveals decreased Msrs activity in a cellular Parkinson's disease model. Our probe affords a powerful tool for detecting and visualizing mitochondrial Msrs in living cells.