Strategies for simultaneous detection and detoxification of Hg2+ using a single sensor from biological and environmental samples are limited and have not been realized in living organisms so far. We report a highly selective, small molecule "turn-on" fluorescent sensor, PYDMSA, based on the cationic dye Pyronin Y (PY) and chelating agent meso-2,3-dimercaptosuccinic acid (DMSA) for the simultaneous detection and detoxification of inorganic mercury (Hg2+). After Hg2+ detection, concomitant detoxification was carried out with sufficient efficacy in living samples, which makes the sensor unique. PYDMSA exhibits high selectivity for Hg2+ over other competing metal ions with an experimental detection limit of ∼300 pM in aqueous buffer solution. When PYDMSA reacts with Hg2+, the CS-C9 bond in the sensor gets cleaved. This results in the "turn-on" response of the fluorescence probe with a concomitant release of one equivalent of water-soluble Hg2+-DMSA complex which leads to a synchronous detoxifying effect. The sensor by itself is nontoxic to cells in culture and has been used to monitor the real-time uptake of Hg2+ in live cells and zebrafish larvae. Thus, PYDMSA is a unique sensor which can be used to detect and detoxify mercury at the same time in living samples. © 2019 American Chemical Society.