Purpose: Purpose of this report is to describe the feasibility of lingual pulse oximetry and lingual near-infrared spectroscopy (NIRS) in a COVID-19 patient to assess lingual tissue viability after several days of mechanical ventilation in the prone position. Materials & Methods: In a COVID-19 ICU-patient, the tongue became grotesquely swollen, hardened and protruding from the oral cavity after 20 h of mechanical ventilation uninterrupted in the prone position. To assess the doubtful viability of the tongue, pulse-oximetric hemoglobin O₂-saturation (SpO₂; Nellcor, OxiMax MAX-NI, Covidien, MA, USA) and NIRS-based, regional tissue O₂-saturation measurements (rSO₂; SenSmart, Nonin, MN, USA) were performed at the tongue. Results: At the tongue, regular pulse-oximetric waveforms with a pulse-oximetric hemoglobin O₂-saturation (SpO₂) of 88% were recorded, i.e. only slightly lower than the SpO₂ reading at the extremities at that time (90%). Lingual NIRS-based rSO₂ measurements yielded stable tissue rSO₂-values of 76-78%, i.e. values expected also in other adequately perfused and oxygenated (muscle-) tissues. Conclusion: Despite the alarming, clinical finding of a grotesquely swollen, rubber-hard tongue and clinical concerns on the adequacy of the tongue perfusion and oxygenation, our measurements of both arterial pulsatility (SpO₂) and NIRS-based tissue oxygenation (rSO₂) suggested adequate perfusion and oxygenation of the tongue, rendering non-vitality of the tongue, e.g. by lingual venous thrombosis, unlikely. To our knowledge, this is the first clinical report of lingual rSO₂ measurement.