Although new spintronic devices based on the giant spin-orbit splitting of single-layer MoS_{2} have been proposed, such splitting has not been studied effectively in experiments. This Letter reports the valence band spin-orbit splitting in single-layer MoS_{2} for the first time, probed by the triply resonant Raman scattering process. We found that upon 325 nm laser irradiation, the second order overtone and combination Raman modes of single-layer MoS_{2} are dramatically enhanced. Such resonant Raman enhancement arises from the electron-two-phonon triple resonance via the deformation potential and Fröhlich interaction. As a sensitive and precise probe for the spin-orbit splitting, the triply resonant Raman scattering will provide a new and independent route to study the spin characteristics of MoS_{2}.