The synthesis and photophysical properties of a novel fluorescent sensor are described. The phosphorus-selenium moiety allowed a selective mercury salt complexation, followed by the formation of phosphane oxide, which leads to a turn-on of the fluorescence. The sensibility and selectivity toward mercury cations were evaluated (0.18 ppb) and found to be in complete adequation with the targeted level of the World Health Organization, which makes the dye an efficient dosimeter for mercury cations.