In the current research, a successful and fast synthesis of gold nanoparticles (AuNPs) by utilizing the leaf extract of Eryngium planum is reported. In order to analyze the influential synthesis parameters, the Plackett-Burman (PB) design was utilized, while the response surface methodology (RSM) was employed for optimization purposes. Furthermore, the achieved AuNPs were applied in the production of the Au-MoS2 nanocomposite. The efficiency of the visible-light photocatalyst was examined in the photocatalytic decolorization of the refractory azo dye, Acid Blue 113 (AB113). A high dye removal efficiency (94%) was observed under the optimal conditions of pH 7.6 using 15.5 mg of Au-MoS2, and 14 min. These results proved that the fabricated Au-MoS2 photocatalyst using biogenic AuNPs was more efficient than pure MoS2 and could be used as a promising catalyst for the degradation of dye-containing effluents.