Spontaneous hemothorax due to anticoagulant use is extremely rare in clinical practice. Dabigatran is a novel anticoagulant to prevent stroke or thromboembolic episodes in patients with nonvalvular atrial fibrillation. We report on an 83-year-old man who received dabigatran therapy (110 mg twice daily) for 7 months and developed massive spontaneous hemothorax and acute renal failure. The patient was admitted to the hospital with complaint of a dull ache in the chest and dyspnea. Chest computed tomography scan revealed massive pleural effusion in the left hemithorax with atelectasis. Acute renal failure was observed 4 days later after admission. Almost 2500 mL of blood was repeatedly drained by ultrasound-guided thoracocentesis, followed by a dramatic decrease in serum red blood cell count, hemoglobin and hematocrit. After excluding other possible causes, diagnostic withdrawal was performed for dabigatran, and plasma transfusion was conducted to supply the lost blood volume. A causal relationship was established, because the patient's renal function gradually improved and no further pleural effusion developed after dabigatran was discontinued. This is a rare case report of massive spontaneous hemothorax caused by dabigatran. Therefore, practitioners should be aware of hemothorax as a potential complication of dabigatran therapy.