Pelvic insufficiency fracture is a very rare complication after total knee arthroplasty (TKA). This fracture occurs in elderly osteoporotic patients, spontaneously or after minor trauma, and its diagnosis can be difficult unless there is a high degree of clinical suspicion. The consequence of missed pelvic insufficiency fracture can be profound: this fracture may progress into an unstable fracture that leads to significant disability. Bone scans assist in the early diagnosis of this fracture. We present a case of osteoporotic pelvic insufficiency fracture after bilateral TKA. A 76 year-old female underwent bilateral TKA for advanced osteoarthritis in her both knees. She also had osteoporosis, and thus we filled an oral bisphosphonate (weekly Risedronate) prescription. Nine months after TKAs, she presented with significant pain around the left buttock and inguinal area without specific trauma. With the use of plain radiographs and a bone scan, a pelvic insufficiency fracture was detected. We successfully treated the patient with conservative management including rest with a pelvic strap, and continued medication for osteoporosis.