Antioxidant peptides have been widely reported, whereas the intracellular antioxidant activity of a tripeptide (Pro-His-Pro, PHP), which was newly isolated and identified from Chinese Baijiu in our previous study, are still poorly understood. This study investigated the protective effects of PHP on 2,2′-azobis (2-methyl-propanimidamidine) dihydrochloride (AAPH)-induced oxidative stress in HepG2 cells and the involved molecular mechanisms. Pretreatment with PHP suppressed the generations of reactive oxygen species (ROS), malondialdehyde (MDA) and oxidized glutathione (GSSG), prevented a decrease in reduced glutathione (GSH), and up-regulated the activities of cellular antioxidant enzymes. Moreover, PHP treatment stimulated the mRNA and protein expression levels of antioxidant enzymes and nuclear factor E2 related factor 2 (Nrf2). Meanwhile, PHP markedly reduced the level of Kelch-like ECH-associated protein 1 (Keap1), suggesting that PHP effectively activated Nrf2/antioxidant response element (ARE)-mediated activity. These findings provide the first molecular basis for the health-promoting effects of PHP to prevent AAPH-induced oxidative stress.