Rosmarinic acid (RA), a food-derived polyphenolic compound, has been reported to possess anti-oxidant, antiallergic, and anti-inflammatory activities. In this study, we investigated whether RA has cytoprotective activity in SIN-1 induced HepG2 cells and how to achieve it. The results show that RA attenuates SIN-1 induced cytotoxicity in HepG2 cells. RA significantly reduces the intracellular reactive oxygen species (ROS) generated from SIN-1 and induced heme oxygenase-1 (HO-1) expression. RA-induced HO-1 expression was attenuated by a pretreatment with specific inhibitors for JNK, ERK, and phosphoinositide 3-kinase (PI3K). The addition of N-acetyl-cysteine (NAC) also reduced HO-1 induction in RA-induced cells. Furthermore, RA induced the translocation of NF-E2-related factor 2 (Nrf2). Taken together, we conclude that RA attenuates cytotoxicity in SIN-1-induced HepG2 cells by direct radical scavenging activity and HO-1 induction through a ROS/PI3K/MAPKs/Nrf2 pathway.