Fatigue stress fractures are quite common in athletes, especially in women. They typically involve the lower extremities, but they have also been described in the pelvis and sacrum. Most of the time, they are radiographically occult at the beginning, and become visible only when the callus is detected. They are suspected owing to their clinical features, and investigated using magnetic resonance imaging (MRI) and computed tomography (CT). We present a case of unusual supero-medial iliac fatigue stress fracture in a 58-year-old amateur marathon runner woman presenting with a history of 1-month's worsening pain in the right buttock. Diagnosis was challenging. The early radiograph was normal, and the stress fracture was suspected by clinical features and MRI. Although the fracture was evident on the MRI, the findings were subtle and overlooked. The fracture was later confirmed by CT and radiograph, showing the reparative process.