Insufficiency fracture is a type of stress fracture, which is the result of normal stresses on abnormal bone. Postmenopausal osteoporosis is the most common cause of insufficiency fractures. An early diagnosis is best made with a bone scan or magnetic resonance imaging, as radiographs may initially appear normal. Insufficiency fractures of the lower leg and ankle are less common. Furthermore, reports of medial malleolar insufficiency fracture without any history of trauma in elderly patients are extremely rare. Thus, we report a case with a medial malleolar insufficiency fracture of the ankle in an elderly patient with osteoporosis. This case shows that we should be aware of the possibility of encountering an uncommon medial malleolar insufficiency fracture as a cause of pain in the ankle region of an elderly patient with osteoporosis.