Myocardial protection during cardiopulmonary bypass relies on intermittent administrations of cardioplegia without continuous, tissue‑level feedback on myocardial oxygen supply and mitochondrial metabolic status. Extended near‑infrared spectroscopy (eNIRS) can enable real-time intramyocardial monitoring of hemoglobin/myoglobin oxygen saturation (S_Hb/Mb) and cytochrome c oxidase oxidation (O_CcO) for assessment of cardiac oxygen availability and utilization during surgery. In this prospective, single-center feasibility study, myocardial eNIRS monitoring was carried out in 15 adults undergoing aortic valve replacement with or without coronary artery bypass grafting. The study focused on initial feasibility and performance. Clinical performance was explored by relating changes in O_CcO responses to intermittently administered blood cardioplegia volumes. Myocardial eNIRS consistently recorded S_Hb/Mb, and O_CcO responses to cross-clamp, cardioplegia, and reperfusion throughout all operations. Mitochondrial oxygenation status improvement (O_CcO area under the curve) correlated with the intermittently administered volume of blood cardioplegia (r = 0.68, p < 0.0001). No device-related adverse events were observed. Feedback on usability was favorable (surgeons 87%; anesthesiologists 85%; nurses 74%). Intraoperative myocardial eNIRS monitoring is feasible. It provides continuous assessment of oxygen availability and mitochondrial redox status during surgery. Real‑time monitoring of ischemia–reperfusion and cardioplegia effects may support intraoperative decision‑making for myocardial protection.