Abstract The present work describes the ultrasensitive and selective spectrofluorimetric determination of Hg(II) using 2,5-dimercaptothiadiazole (DMT) as a fluorophore. DMT shows an emission maximum at 435 nm while exciting at 330 nm. The colorless solution of DMT changes into a highly emittive yellow color immediately after the addition of 0.5 μM Hg(II) and nearly 245-fold increase in emission intensity at 435 nm was observed. These changes were ascribed to the complex formation between Hg(II) and DMT. Based on the fluorescence enhancement, the concentration of Hg(II) was determined. The binding constant value ( K A =1.8620×10 4 mol −1 L) suggests that there is a strong binding force between Hg(II) and DMT. The fluorescence quantum yield of DMT–Hg(II) complex was found to be 4-fold higher than that of DMT, indicating that the DMT–Hg(II) complex was highly emittive than the DMT. Interestingly, the emission intensity was increased even in the presence of 0.1 pM Hg(II). The fluorophore showed an extreme selectivity towards 100 nM Hg(II) in the presence of 50,000-fold higher concentrations of Na +, K +, Ca 2+, Mg 2+, Fe 2+, Fe 3+, Cd 2+, Cr 3+, Mn 2+, Zn 2+, Co 2+, Ni 2+, Cl −, SO 4 2−, NO 3 − ions and 1000-, 500- and 200-fold higher concentrations of Cu 2+, Pb 2+ and Ag + ions, respectively, as interferences. The lowest detection of 18 pg L −1 Hg(II) (LOD=3 S/ m) was achieved for the first time using DMT by fluorimetry. The proposed method was successfully utilized for the determination of Hg(II) in tap water, river water and industrial waste water samples.