We present a 16-year-old boy with mild hemophilia A who developed superficial fibromatosis mimicking subcutaneous hematoma of the right upper dorsal region of the foot. This condition is very rare. Hematoma originally developed after the patient sprained the right ankle. Although cast immobilization and replacement therapy with factor VIII (FVIII) concentrates reduced the symptoms, the swelling and pain reappeared in the same region 4 weeks after the cast was removed. A clinical diagnosis of intramuscular hemorrhage was made. Despite readministration of regular FVIII concentrates and cast immobilization, the patient experienced a relapse after removal of the second cast. Magnetic resonance imaging demonstrated a tumor in the extensor digitorum brevis muscle of the right foot with high intensity on T2-weighted images. Marginal excision resulted in symptom resolution. A histopathologic examination of the tissue showed proliferation of fibroblastic cells with abundant collagen bundles. When a mass is found under the skin in hemophilia A patients, an etiology other than bleeding can be overlooked because subcutaneous hemorrhage in limited areas often results in a mass; however, the possibility of rare soft tissue tumors in hemophilia patients should be kept in mind.