We report a 19-year-old woman with a 6-month history of nontraumatic left foot numbness associated with intermittent weakness. Nerve conduction studies and electromyography localized the lesion to the posterior tibial nerve, below the innervation to the soleus and medial gastrocnemius muscles. MRI of the left leg revealed a multiloculated cystic collection near the proximal tibiofibular joint. Surgical excision and pathology confirmed the diagnosis of a ganglion cyst, in an atypical location distal to the popliteal fossa. We believe this is the first reported case of fascicular posterior tibial nerve involvement by a ganglion cyst in the calf.