In this work, a new method was developed for the determination of two phenothiazine drugs, namely thioridazine hydrochloride andchlorpromazine hydrochloride using gold nanoparticles-based resonance light scattering (RLS) technique. The addition of these drugsto gold nanoparticles solution in acidic medium leads to the aggregation of nanoparticles, which, in turn, causes a marked enhancementin RLS intensity at 400 nm. The aggregates were characterized by UV-Vis spectroscopy and transmission electron microscopy. Theeffects of important experimental factors such as pH, concentration of gold nanoparticles and incubation time on RLS signal were investigatedand optimized. Under the optimum conditions, the calibration graphs were linear in the range of 3.6 - 36 and 3.5 - 30 ng/mL, forthioridazine hydrochloride and chlorpromazine hydrochloride, respectively with corresponding limits of detection of 1.1 and 1.0 ng/mL.The results indicate that the nanogold-based RLS is a very sensitive and simple method for the determination of phenothiazine drugs inpharmaceutical samples and human plasma.