BACKGROUND This article presents a rare case of an intra-tendinous ganglion cyst of the peroneus tertius. Ganglion cysts are benign lesions frequently seen in hand pathologies, but they are rarely seen in the foot and ankle. This article discusses the present case and similar previously reported cases in the English literature. CASE REPORT We present a case of a 58-year-old man with a 3-year history of right foot pain caused by a mass located at the dorso-lateral aspect of the midfoot. Preoperative MRI demonstrated a ganglion cyst arising from the peroneus tertius tendon sheath. The lesion was successfully decompressed in the office; however, it recurred 7 months later. As it was symptomatic, we elected to proceed with surgical resection. During dissection, it became apparent that the cyst was arising from an intrasubstance tear of the peroneus tertius tendon, and a branch of the superficial peroneal nerve was noted to be adherent to the pseudo-capsule. Following excision of the lesion and its expansile pseudo-capsule, the tear was repaired with tubularization of the tendon and external neurolysis of the nerve was performed. At 6 months after surgery, there was no recurrence of the lesion, and the patient was pain free and had regained normal physical function. CONCLUSIONS Intra-tendinous ganglion cysts are rare, especially in the foot and ankle. This makes it challenging for an accurate preoperative diagnosis. When a tendon is arising from a tendon sheath, we recommend exploration of the underlying tendon for an associated tear.