Synovial or ganglion cysts of the proximal tibiofibular joint are less common than synovial cysts of the knee joint but may present in a similar manner and may be difficult to diagnose clinically. Although synovial cysts arising from the knee joint after prosthetic arthroplasty have already been described, we report a case in which a lateral knee mass compressing the peroneal nerve was found to be a synovial cyst arising from the tibiofibular joint.