In the present study, a new type of selenium nanoparticles (SeNPs) was synthesized using oyster protein hydrolysates (OPH) as both a stabilizer and capping agent upon ultrasonication. OPH with alcohol dehydrogenase activation activity was prepared and used to fabricate OPH-SeNPs, where monodisperse SeNPs with a transparent orange appearance, ranging from 140 to 310 nm, were obtained. Physicochemical analysis further suggested a weak interaction between SeNPs and the –NH, C=O, COO–, and C–N groups of the OPH, and the formed nanocomposite with improved stability against aggregation was in an amorphous state. The hepatoprotective effect of OPH-SeNPs on HepG2 cells showed that pre-incubation with OPH-SeNPs significantly decreased cell apoptosis induced by hydrogen peroxide and could well maintain cell integrity. A reduction in intracellular reactive oxygen species was found, along with ameliorated activity of internal antioxidant enzymes. The observed upregulation of key antioxidant-related components, i.e., glutathione peroxidase, nuclear factor erythroid 2-related factor 2 (Nrf2), and heme oxygenase-1, suggests activation of the Nrf2-antioxidant response element signaling pathway, which mechanistically explains the hepatoprotective effect of OPH-SeNPs against hydrogen peroxide-induced cytotoxicity in HepG2 cells.