In this study, we investigated the protective effects of the anthocyanin cyanidin-3-O-β-glucoside (C3G) on hypercholesterolemia-induced endothelial dysfunction in apoE-deficient (apoE(-/-)) mice. In the prevention study, twenty 8-wk-old male apoE(-/-) mice (n = 10/group) were fed a high-fat, cholesterol-rich diet (HCD) or the HCD supplemented with C3G (2 g/kg diet) for 8 wk. The endothelium-dependent relaxation response to acetylcholine in the aortas of the C3G-fed mice was greater compared with those fed the HCD (P < 0.05). The atherosclerotic plaque area in the aortic sinus of mice fed the C3G diet was lowered by 54% compared with those fed the HCD (P < 0.01). Mice fed C3G had greater expression of the ATP-binding cassette transporter G1 (ABCG1) and lower cholesterol, mainly 7-ketocholesterol (7-KC), concentrations than those fed the HCD. Superoxide production and lipid hydroperoxides in aorta were lower in mice fed C3G compared with those fed the HCD. The phosphorylation levels at Ser1177 of endothelial NO synthase (eNOS) and the production of cyclic GMP (cGMP) in aorta were greater in C3G-fed mice than in HCD-fed mice. In the therapy study, apoE(-/-) mice were fed the HCD for 8 wk and then continued to receive the HCD or were switched to the HCD supplemented with C3G (2 g/kg diet) for another 8 wk. The established endothelial dysfunction and atherosclerosis were reversed, accompanied by greater ABCG1 expression in aorta, lower cholesterol and 7-KC concentrations, and greater generation of cGMP in mice fed C3G compared with those fed the HCD. Taken together, our results show that the anthocyanin C3G prevents or reverses hypercholesterolemia-induced endothelial dysfunction by inhibiting cholesterol and 7-oxysterol accumulation in the aorta and the subsequent decrease in superoxide production, thereby preserving eNOS activity and NO bioavailability.