A benzothiazolium-derived colorimetric and fluorescent chemosensor (L) for mercury ions has been prepared. The detection limits of the colorimetric and fluorescent method are 15.3 and 11.8 ppb, respectively, much lower than the reported chemosensors based on similar derivatives of benzothiazolium. The optimized molecular structures, absorption and fluorescence characteristics of the chemosensor and its complex with mercury ions were performed using calculations at the B3LYP/LanL2DZ level of theory, in combination with Atoms in Molecules and Natural Bond Orbitals analyses.