Aqueous solutions of a small size electrolyte, sodium p-toluenesulfonate (NapTS), were examined for dielectric relaxation behavior. Dissociated pTS(-) shows pronounced dielectric relaxation behavior in the aqueous solutions well-described with a Debye-type relaxation function. The value of the dielectric relaxation time is much longer than that of water molecules and is very close to the rotational relaxation time determined from the value of a spin-lattice relaxation time (T-1, H-1 NMR). This means that pTS(-) keeps significantly large dipole moment (mu) and its rotational relaxation mode is detected by the dielectric relaxation measurements. Moreover, the concentration dependence of dielectric relaxation data suggests that 2-3.5 water molecules are tightly hydrated to pTS- in aqueous solution. We also carried out ab initio quantum chemical calculations to obtain the optimized geometries of individual pTS(-), pTS(-) dihydrate, and pTS(-) trihydrate. The magnitudes of mu evaluated to be 7.3, 6.1, and 6.2 D, respectively, at the optimized geometries are significantly large and are crucial evidence that pTS(-) keeps large mu in aqueous solution.