We found a rare muscular variation in the superficial region of the popliteal fossa in a 61-year-old Korean male cadaver whose cause of death was laryngeal carcinoma during routine dissection course for medical students. The muscle ran transversely between the medial head of the gastrocnemius muscle and the tendon of the long head of biceps femoris muscle, covering the neurovascular structures in the popliteal fossa. The muscle received its nerve supply from the tibial nerve. Based on its innervation, we speculated that the anomalous muscle might be a very specific type of variation related to the gastrocnemius tertius rather than another superficial muscle in the popliteal fossa.