Reactive oxygen species detoxification potentials of Sorghum bicolor polyphenolic extract was investigated in the liver of N-nitrosodiethylamine-treated rats. Male rats, weighing (135 ± 5.5) g were completely randomized into 7 groups (A–G) of five rats each. Rats in C, D, E and F were administered orally once daily at 24-h interval for 7 d with 500, 125, 250 and 500 mg/kg body weight of polyphenolic extract of S. bicolor, respectively. Group G was given 100 mg/kg body weight of vitamin C. On the sixth day, groups B, D, E, F and G were administered with 100 mg/kg body weight N-nitrosodiethylamine (NDEA). Group A, which served as the control was treated like the test groups except, that the animals received distilled water only. Reactive oxygen species detoxifying enzymes (superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glucose 6-phosphate dehydrogenase) activities were significantly (P < 0.05) induced by S. bicolor. These inductions significantly (P < 0.05) attenuated the NDEA-mediated decrease in reactive oxygen species detoxifying enzymes and compared favourably with vitamin C. NDEA-mediated elevation in the concentrations of oxidative stress biomarkers; malondialdehyde, conjugated dienes, lipid hydroperoxides, protein carbonyl and percentage DNA fragmentation were significantly (P < 0.05) lowered by S. bicolor polyphenolic extract. Overall, the results obtained from this study revealed that the polyphenolic extract of S. bicolor grains enhanced the detoxification of reactive oxygen species in NDEA-treated rats. The polyphenols also prevented the peroxidation of lipid, oxidation of proteins as well as fragmentation of DNA component in the liver of rats and hence gave the evidence of possible prophylactic potentials of S. bicolor grains.