The synthesis, photophysical properties, and Hg(II) binding of a red-emitting sensor for mercuric ion are presented. 2-{11-[(2-{[Bis-(2-ethylsulfanylethyl)amino]methyl}phenylamino)methyl]-3 -hydroxy-10-oxo-10H-benzo[c]xanthen-7-yl}benzoic acid (MS5) is based on the seminaphthofluorescein chromophore and employs a thioether-rich metal-binding unit. This sensor affords both turn-on and single-excitation dual-emission ratiometric Hg(II) detection in aqueous solution. The fluorescence response of MS5 is Hg(II)-specific, and the probe is selective for Hg(II) over alkali and alkaline earth metals, most divalent first-row transition metal ions, and the Group 12 congeners Zn(II) and Cd(II). MS5 binds Hg(II) reversibly and can be recycled. The EC50 for 1 mu M MS5 is 910 nM, and a lower detection limit of 50 nM is obtained when employing 500 nM probe. X-ray crystallographic studies using a salicylaldehyde-based model of MS5 are also presented. 2-[(2-{[Bis-(2-ethylsulfanylethyl)amine]methyl}phenylamine)methyl]phenol coordinates Hg(II) with two thioether sulfur atoms, two amino nitrogen atoms, and a phenol oxygen atom arranged in a distorted trigonal bipyramidal geometry. Studies of natural water samples spiked with mercuric salts indicate that MS5 can rapidly detect Hg(II) in such complex solutions and demonstrate its potential utility in the field.