We examined the suppressive effect on lipid peroxidation of electrolyzed reduced water (ERW), which exhibited reactive oxygen species (ROS)-scavenging activity in vitro and in cultured cells. The oxidation of linoleic acid by 2, 2’-azobis (2-amidinopropane) hydrochloride (AAPH) was suppressed by ERW in vitro. ERW also suppressed the lipid peroxidation in cell membrane of mouse lymphoblast YAC-1 cells. Transparent electron microscopic analysis revealed that platinum in ERW existed as a colloidal form of nm size. Concentrated ERW suppressed the lipid peroxidation in a manner dependent upon the concentration of platinum nanocolloids, and chemical synthesized platinum nanocolloids exhibited a similar suppressive effect on lipid peroxidation to that of ERW, suggesting that platinum nanocolloids contained in ERW were reducing agents to suppress the lipid peroxidation.