Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that contributes a crucial role in protection against ischemia (ISC)-reperfusion (REP) injury by driving expression of anti-apoptotic and anti-oxidant genes. STAT3 is also present in the mitochondria, where it modulates the activity of the electron transport chain (ETC) and the permeability transition pore. Transgenic mice that overexpress a mitochondrial-targeted, transcriptionally inactive STAT3 in cardiomyocytes (MLS-STAT3E mice) exhibit a persistent, partial blockade of electron transfer through complex I that uniquely did not lead to tissue dysfunction at baseline, yet increased mitochondrial ischemic tolerance. The direct contribution of non-transcriptional, mitochondria-localized STAT3 to protection during ISC-REP remains to be established. We hypothesized that the enhanced mitochondrial tolerance to ischemia present in MLS-STAT3E mice would decrease cardiac injury during ISC-REP. In the isolated buffer-perfused heart model, MLS-STAT3E hearts exhibit a decreased infarct size compared to non-transgenic littermate hearts. Contractile recovery, expressed as a percent of LV developed pressure before ISC, is improved in MLS-STAT3E mice. Mitochondria isolated at the end of 60 min. of REP from MLS-STAT3E hearts show attenuated ROS release. The partial and persistent blockade of complex I present in MLS-STAT3E mice decreases cardiac injury during REP, in part via a persistent decrease in ROS production and attenuation of mitochondrial permeability transition pore opening at the onset of REP. In vivo, MLS-STAT3E hearts exhibit substantially higher postoperative survival rate and a substantial decrease in myocardial infarct size. STAT3 mediates cardio-protection not only via canonical action as a transcription factor, but also as a modulator of ETC activity directly in the mitochondria.