A rectus abdominis sheath hematoma (RSH) is uncommon. It may mimic other acute abdominal disorders. The underlying conditions are trauma, coagulation disorders, or anticoagulant therapy, complications related to operations, subcutaneous injections to the abdominal wall, although it can also develop spontaneously. Acute abdominal pain and a palpable mass after muscular strain such as coughing, sneezing, and twisting were features highly suggestive of RSH. The diagnostic means of choice is computerized tomography. The treatment is usually conservative, but surgery may be needed in cases with large or progressing hematomas or with severe symptoms. We herein report an elderly woman presenting with an acute painful abdominal mass, without any underlying conditions, which was diagnosed as spontaneous RSH. She needed an operation. We concluded that RSH should be considered in the differential diagnosis of acute abdominal pain in the elderly, even in the absence of underlying conditions.