Oxidative stress is closely associated to the onset and progression of many human diseases. Activation of the nuclear factor erythroid 2-related factor 2 (Nrf2)/antioxidant response element (ARE) signaling pathway using naturally-derived molecules is an efficient strategy for alleviating the intracellular oxidative insults, and thus blocking the pathogenesis of oxidative stress-induced diseases. In the present study, a naturally-derived isopimarane-type diterpenoid sphaeropsidin C (SC) was identified to be an activator of Nrf2/ARE signaling pathway. Our data indicated that SC was able to stimulate Nrf2-mediated defensive system through promoting Nrf2 translocation, inhibiting Nrf2 ubiquitination, and enhancing Nrf2 stability in normal human lung epithelial Beas-2B cells. Furthermore, SC-induced Nrf2 activation required the involvement of protein kinases, exemplified by protein kinase C (PKC), protein kinase R-like endoplasmic reticulum kinase (PERK), and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K). It alleviated sodium arsenite [As(III)]-induced intracellular oxidative stress in an Nrf2-dependent manner. These results suggested that SC displayed potential application for the prevention and therapy against oxidative stress-induced diseases. Moreover, isopimarane-type diterpenoid represents a promising skeleton for developing Nrf2 activators.