This article reports the clinical and histopathological appearance of synovial osteochondromatosis in the right knee joint of a 9-year-old girl. Initial symptoms such as monoarticular joint swelling led to further diagnostic procedures with the aid of different imaging techniques including X-ray examination and MRI. The definitive diagnosis was based on histopathological findings after performing arthroscopy of the affected joint and the intraoperative appearance of the synovial lesions. Besides antiphlogistical drug-therapy, the treatment of choice is synovectomy.