This case report clarifies an adverse reaction of antiplatelet therapy which has been a standard prophylactic method for patients harboring significant risks of thromboembolic events. A 71-year-old Japanese man who had been taking aspirin tablets (81 mg) for a year presented with sudden colic pain in the left flank region. An abdominal computed tomography scan revealed a significant perirenal hematoma of the left kidney. There were no pathological kidney conditions, such as renal tumors, calculi or vascular diseases, found by magnetic resonance imaging examination. After cessation of aspirin administration followed by conservative management, the hematoma completely disappeared 6 months later. This is the first documented case of spontaneous perirenal hematoma secondary to low-dose aspirin treatment. While such unpleasant events occur extraordinarily, this should be noted as a severe risk of antiplatelet therapy.