BACKGROUND: Since acute respiratory failure (ARF) is a life-threatening complication, particularly in the gestational period, differential diagnosis and rapid treatment are required. Among the various causes of sudden onset of ARF, thyroid storm is a rare cause in a parturient complicated with well-controlled hyperthyroidism. In this case report, we describe a parturient with hyperthyroidism in whom a thyroid storm manifesting congestive heart failure and pulmonary edema developed just before an emergency ceasarean section, even though hyperthyroidism was well-controlled with antithyroid drugs. CASE PRESENTATION: A 36-year-old pregnant woman was diagnosed as having clinical chorioamnionitis, and an emergency cesarean section was performed at 25 weeks of pregnancy. She had a complication of hyperthyroidism accompanied by mild mitral regurgitation, and she had been treated with methimazole. She was treated with ritodrine and MgSO4 for the threat of premature delivery. At the preoperative consultation, her percutaneous oxygen saturation (SpO2) was 98% on room air. When she was admitted to the operating room, her heart rate and blood pressure were 130 beats/min and 196/78 mmHg, respectively. SpO2 was 88% on room air without any symptoms; however, just after starting oxygen administration via a facemask, she complained of severe respiratory distress and became agitated. Partial pressure of arterial oxygen was 108 mmHg with an inspiratory oxygen fraction of 1.0. Chest radiography revealed pulmonary congestion, and transesophageal echocardiography revealed normal right ventricular function without an embolus and severe mitral regurgitation with preserved left ventricular function. Contrast-enhanced computed tomography after the operation revealed no pulmonary embolus but revealed a pulmonary effusion, and free triiodothyronine level was increased at the onset of dyspnea. Therefore, we diagnosed the causes of sudden onset of dyspnea as pulmonary edema and congestive heart failure induced by a thyroid storm. CONCLUSION: Sudden onset of a thyroid storm just before a cesarean section occurred in a patient with several risk factors of thyroid storm and pulmonary edema, including pregnancy, treatment with tocolytic agents, and infection. The involvement of these multiple factors was considered to be the cause of the sudden onset of the thyroid storm and the cause of rapidly progressive pulmonary edema.