Whilst a few studies have associated various symptoms with the presence of a peroneus quartus muscle in the peroneal compartment of the leg, little is known of the clinical relevance of this muscle. We dissected 102 cadaver legs and reviewed the magnetic resonance images of 80 patients with symptoms from the ankle. The peroneus quartus, with a number of different attachments, was present in 6.6% of the legs. It most commonly arose from the peroneus brevis muscle and inserted into the retrotrochlear eminence of the calcaneum. Associated pathology included a longitudinal tear in the tendon of peroneus brevis, possible peroneal tendon subluxation or dislocation, and a prominent retrotrochlear eminence. On the MR scans its presence was associated with pain and weakness of the ankle. Orthopaedic surgeons and radiologists should be aware of the possible presence of the peroneus quartus muscle, not only because of possible associated pathology, but also for its potential use for surgical reconstruction.