Systemic‐to‐pulmonary venous shunting is a rare cause of refractory hypoxemia. Its prompt recognition is crucial, as management requires not only etiological treatment but also the restoration of physiological venous return. We report the case of a 40‐year‐old man presenting with dyspnea, cervicothoracic edema, and hypoxemia. Imaging revealed a mediastinal mass compressing the superior vena cava (SVC), with associated subocclusive thrombosis in its lower segment and right pulmonary artery. Endobronchial ultrasound‐guided puncture and video‐assisted thoracoscopic marsupialization confirmed a benign bronchogenic cyst. Following anesthesia induction, the patient developed refractory hypoxemia requiring Veno‐Arterial Extracorporeal Membrane Oxygenation, chosen due to prohibitive obstruction of the SVC preventing veno‐venous access. Persistent hypoxemia prompted further investigations, revealing a right‐to‐left shunt through a mediastinal venous cavernoma connecting upper‐body venous return to the pulmonary veins. Mechanical thrombectomy of the SVC using the ClotTriever system restored venous patency, allowing ECMO weaning and full clinical recovery. This case highlights an exceptional systemic‐to‐pulmonary venous shunt. Successful management required a multimodal strategy integrating bronchoscopic, surgical, and endovascular interventions with ECMO support. Clinicians should consider extracardiac venous shunting as a rare cause of refractory hypoxemia and select the optimal ECMO configuration after careful evaluation of both hemodynamic status and vascular anatomy.