Carnosic acid (CA), found in rosemary, has been reported to have antioxidant and antiadipogenic properties. Here, we investigate the molecular mechanism by which CA inhibits hydrogen peroxide (H₂O₂)-induced injury in HepG2 cells. Cells were pretreated with 2.5–10 μmol/L CA for 2 h and then exposed to 3 mmol/L H₂O₂ for an additional 4 h. CA dose-dependently increased cell viability and decreased lactate dehydrogenase activities. Pretreatment with CA completely attenuated the inhibited expression of manganese superoxide dismutase (MnSOD) and the B-cell lymphoma-extra large (Bcl-xL), and reduced glutathione activity caused by H₂O₂, whereas it reversed reactive oxygen species accumulation and the increase in cleaved caspase-3. Importantly, sirtuin 1 (SIRT1), a NAD⁺-dependent deacetylase, was significantly increased by CA. Considering the above results, we hypothesized that SIRT1 may play important roles in the protective effects of CA in injury induced by H₂O₂. As expected, SIRT1 suppression by Ex527 (6-chloro-2,3,4,9-tetrahydro-1H-carbazole-1-carboxamide) and siRNA-mediated SIRT1 silencing (si-SIRT1) significantly aggravated the H₂O₂-induced increased level of cleaved caspase-3 but greatly reduced the decreased expression of MnSOD and Bcl-xL. Furthermore, the positive regulatory effect of CA was inhibited by si-SIRT1. Collectively, the present study indicated that CA can alleviate H₂O₂-induced hepatocyte damage through the SIRT1 pathway.