This work demonstrates the development of a highly sensitive method to detect and quantify sulfide ions (S^2–) in water samples. First, we synthesized 6-mercapto-s-triazolo(4,3-b)-s-tetrazine (MTT) by the reaction between formaldehyde and 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole at room temperature. The synthetic MTT was used as a capping ligand for the synthesis of gold nanodots (AuNDs) via a one-pot green method at room temperature with only a 10 min reaction time. Transmission electron microscopy images exhibited that the MTT-AuNDs have an average particle size of 1.9 nm and an emission maximum at 672 nm upon excitation at 360 nm. The synthesized highly red emissive MTT-AuNDs are used as specific fluorescent probes for the detection of S^2–. The fluorescence of MTT-AuNDs was significantly and dose-dependently quenched by the addition of S^2–. The observed fluorescence quenching was ascribed to the formation of an Au₂S complex, which was determined by Raman and mass spectroscopy. A good linearity was achieved for the increasing concentration of S^2– from 870 nM to 16 μM, and the detection limit was found to be 2 nM (S/N = 3). The S^2– detection system that is described in this study was validated and agreed well with the standard methylene blue method. Furthermore, the present sensor was examined for its use in quantifying S^2– in real water samples obtained from lakes and rivers. In addition, the specificity was checked against the most likely ion interferences in real water. Moreover, a cost-effective and viable paper-based S^2– sensor was fabricated for environmental monitoring based on the use of MTT-AuNDs. The developed system would be an environmentally friendly and easy-to-use detection device for S^2– in water.