Multispectroscopic techniques were used to investigate the interaction of S-allyl cysteine (SAC) from garlic with human serum albumin (HSA). UV-Vis absorption measurements prove the formation of the HSA-SAC complex. An analysis of fluorescence spectra revealed that in the presence of SAC, the quenching mechanism of HSA is considered static. The quenching rate constant Kq, KSV, and the binding constant KA were estimated. According to the Van't Hoff equation, the thermodynamic parameters enthalpy change (ΔH) and entropy change (ΔS) were calculated to be -1.00×105 J/mol and -255 J/mol/K, respectively. These indicate that hydrogen bonds and van der Waals forces are the major forces between SAC and HSA. The changes in the secondary structure of HSA, which was induced by SAC, were determined by circular dichroism spectroscopy. Energy transfer was confirmed and the distance between donor and acceptor was calculated to be 2.83 nm.