Peroneal tendon dislocation is an uncommon injury that is sometimes associated with peroneal anomalies. The lowlying peroneus brevis muscle belly, defined as an abnormal distal muscle belly extension into the retromalleolar groove, is rarely reported and may allow peroneal tendon dislocation by stretching out the superior peroneal retinaculum. Here we report the case of a 20-year-old male patient with peroneal tendon dislocation combined with a low-lying peroneus brevis muscle belly. Peroneal tendon dislocation was treated with superior peroneal retinaculum reinforcement and abnormal muscle belly debulking. The clinical outcome was satisfactory and showed no peroneal tendon dislocation recurrence 9 months postoperatively.