Metallosis is a rare cause of failure after unicompartmental knee replacement (UKR) and has only previously been reported when there has been abnormal metal-on-metal contact. We present a case report of a 67-year-old woman who was admitted to our department with a 5-month history of a painful right knee. There was no history of trauma and her past medical history was unremarkable. She had undergone a primary mobile bearing medial UKR of her right knee 5 years ago. During the preceding 5 months, the patient complained that the knee was becoming unstable, was painful, and was making frequent clicking noises associated with metallic friction. Plain radiographs of the right knee showed dislocation of the polyethylene bearing and the patient was taken to the operating room for revision of the implant. Intraoperatively, severe evidence of metallosis was found in the periprosthetic soft tissues. The tibial component was found to be loose and the polyethylene bearing was dislocated posteriorly. The implants were revised. Postoperative recovery was uneventful. Microscopic examination revealed findings consistent with metallosis. At the latest follow-up, the patient had a good clinical and radiological outcome. In conclusion, such a complication of UKR requires early diagnosis and treatment as it can result in implant loosening and metallosis.