The detection of biothiols has attracted extensive interest owing to their vital importance in maintaining bioprocesses. Here, we reported a fluorescent probe MANK with facile preparation and excellent water solubility, which can effectively distinguish Cys from its two analogues glutathione (GSH) and homocysteine (Hcy) with a large “turn‐on” fluorescent signal of more than 40‐fold enhancement. Importantly, during the selective detection of Cys, the large concentration differences of three biothiols in living cells had been taken into account. The maleyl group in MANK acted both as a recognition site and fluorescence quenching group. And in the presence of Cys, the thiol group of Cys reacted with maleyl group of MANK by nucleophilic addition and the resulting adduct restored fluorescence emission with a high fluorescent quantum yield of 0.40 in phosphate buffered saline (PBS). Finally, it was successfully utilized to map the endogenous Cys in living cells by confocal microscopy.