Snapping knee syndrome describes the sudden movement of a soft-tissue structure across a bony prominence around the knee with a popping sound at some specific activity. Symptomatic snapping knee syndrome may result from an intra-articular tumor, but this situation is rare. To the best of our knowledge, an intra-articular ganglion cyst leading to snapping knee has not been reported previously. A 20 year old female with painful snapping knee was successfully treated by open en-bloc excision of the ganglion cyst sized 4.5 x 1.5 x 1 cm on the ligamentum mucosum. Snapping and pain were completely relieved after surgery. Although the snapping knee is not common over the antero-lateral portion of the knee joint, a ganglion cyst from the ligamentum mucosum is a possible cause.