Eisenmenger's syndrome (ES) is the most severe phenotype of pulmonary arterial hypertension (PAH) secondary to congenital heart disease. In these cases, a significant systemic-to-pulmonary (left-to-right) shunting triggers the development of pulmonary vascular disease (PVD) and pulmonary hypertension. In cases of acute hypoxemic respiratory failure in patients with ES, high flow nasal cannula (HFNC) oxygen therapy should be considered as a first-line approach in order to avoid pulmonary complications and right ventricular overload related to positive pressure ventilation. Here, we report a case of HFNC use in a patient with COVID-19 infection and ES.