The diagnosis of pulmonary thromboembolism (PE) is often delayed because it is usually misdiagnosed as pneumonia or deep vein thrombosis. We report an unusual case of PE misdiagnosed as viral pleuritis on the first arrival at the emergency department (ED) in our hospital. A 14-year-old girl with no previous significant medical history was referred to the ED with pleuritic and chest pain with low-grade fever 4 days before admission. Echography showed a small amount of left pleural effusion. A 12-lead electrocardiogram was normal. She received a diagnosis of viral pleuritis. Two days before admission, she revisited ED with dyspnea and exacerbated pain. Echography showed slight increase in left pleural effusion. She had the same diagnosis. The chest pain remained at the same level. On the day of admission, she presented to ED with vomiting, watery diarrhea, abdominal pain, chest pain, and respiratory distress. Laboratory findings showed hypoalbuminemia and proteinuria. Echography showed a moderate amount of pleural effusion on both sides and no dilatation of the right cardiac ventricle. Contrast-enhanced chest computed tomography was performed to search the cause of the respiratory distress, which showed filling defects with contrast material in pulmonary arteries. A 12-lead electrocardiogram showed an S1Q3T3 pattern. She received a diagnosis of PE caused by nephrotic syndrome. Pulmonary thromboembolism can mimic infectious pleuritis and lead to misdiagnosis and/or delayed diagnosis. Thus, risk factors of PE should be considered in pediatric patients presenting with symptoms suggesting infectious pleuritis.