The Fourier transform-infrared spectrum of 3-methylsulfanilic acid was recorded and analyzed. The vibrational wavenumbers of the compound have been computed using the Hartree-Fock/6-31G* basis and compared with the experimental values. The first hyperpolarizability, predicted infrared intensities and Raman activities are reported. The calculated first hyperpolarizability is comparable with the reported values of similar structures and is an attractive object for future studies of non linear optics. Optimized geometrical parameters of the title compound are in agreement with similar reported structures.