We report a rare case of a 'giant ganglion' with 24 x 10 x 12 cm dimensions originating from the semimembranosus tendon. The patient presented with chronic pain and a palpable mass in his left calf located between the superior aspect of the popliteal fossa and the distal third of the calf. MRI revealed the mass to be a ganglion in close relation to the semimembranosus muscle at its attachment to the tibia. The patient was operated on and had complete resolution of symptoms postoperatively. To the best of our knowledge there are no other case reports in the literature of ganglion cysts of similar size arising from the tendon of semimembranosus. A brief review of the literature is included.