Cardiovascular disease, a disease caused by many pathogenic factors, is one of the most common causes of death worldwide, and oxidative stress plays a major role in its pathophysiology. Tanshinone I (Tan I), a natural compound with cardiovascular protective effects, is one of the main active compounds extracted from Salvia miltiorrhiza. Here, we investigated whether Tan I could attenuate oxidative stress and oxidative stress–induced cardiomyocyte apoptosis through Nrf2/MAPK signaling in vivo and in vitro. We found that Tan I treatment protected cardiomyocytes against oxidative stress and oxidative stress–induced apoptosis, based on the detection of relevant oxidation indexes such as reactive oxygen species, superoxide dismutase, malondialdehyde, and apoptosis, including cell viability and apoptosis-related protein expression. We further examined the mechanisms underlying these effects, determining that Tan I activated nuclear factor erythroid 2 (NFE2)–related factor 2 (Nrf2) transcription into the nucleus and dose-dependently promoted the expression of Nrf2, while inhibiting MAPK signaling activation, including P38 MAPK, SAPK/JNK, and ERK1/2. Nrf2 inhibitors in H9C2 cells and Nrf2 knockout mice demonstrated aggravated oxidative stress and oxidative stress–induced cardiomyocyte injury; Tan I treatment suppressed these effects in H9C2 cells; however, its protective effect was inhibited in Nrf2 knockout mice. Additionally, the analysis of surface plasmon resonance demonstrated that Tan I could directly target Nrf2 and act as a potential Nrf2 agonist. Collectively, these data strongly indicated that Tan I might inhibit oxidative stress and oxidative stress–induced cardiomyocyte injury through modulation of Nrf2 signaling, thus supporting the potential therapeutic application of Tan I for oxidative stress–induced CVDs.