Pneumothorax (PTX) is rarely reported in patients receiving hyperbaric oxygen (HBO2) therapy. Patients with air-trapping lesions in the lungs and those with a history of spontaneous PTX, lung disease, mechanical ventilation or chest trauma are at an increased risk for PTX during HBO2 therapy. A 28-year-old male earthquake survivor was referred to our center for multiple wounds 21 days after being rescued from the debris. He had been intubated and put on mechanical ventilation for three days because of adult respiratory distress syndrome (ARDS). At initial presentation, he was conscious, well-oriented and hemodynamically stable. The initial six HBO2 treatments were uneventful. On the seventh HBO2 treatment, the patient lost consciousness and developed cardiopulmonary arrest near the end of decompression. The HBO2 specialist accompanying the patient inside the chamber immediately initiated CPR. A diagnosis of tension PTX was made. After the patient was removed from the chamber, a chest tube was inserted, which improved the symptoms. Although rare, tension PTX can occur during HBO2 therapy. Early diagnosis and intervention are crucial for saving a patient's life. Increased vigilance is required during treatment of patients with risk factors for PTX.