We report the origins and types of reductant in the discharge-plasma-induced gold nanoparticle (AuNP) synthesis. The formation of AuNPs by contacting Au (III)-containing solution with plasma is ascribed to two types of reductant generated by plasma-solution interaction, ie, the short-lived reductants including free, secondary, and hydrated electrons, hydrogen as well as ultraviolet light, and a long-lived reductant, hydrogen peroxide which has normally been regarded as an oxidant in plasma-induced metal nanoparticle synthesis. We expect that the elucidation of the reductants involved will be beneficial for further understanding and control of plasma-induced metal nanoparticle synthesis.