Muscle hernia (MH) is a focal protrusion of muscle into the overlying subcutaneous tissue through an acquired or congenital facial defect. Herniation in the thigh muscles is not common and there are limited reports evaluating this pathology using ultrasonography (US). A 28-year-old amateur football player presented with a palpable mass on the anterior aspect of his right thigh. Sonographic examination demonstrated a MH through a fascial defect, which is called mushroom appearance. Based on the clinical and sonographic findings, the diagnosis of a MH in the rectus femoris muscle was established. Today, there is no doubt anymore on the role of musculoskeletal US in the field of sports injuries. Therefore, with its various advantages (non-invasive, practical and convenient, inexpensive, etc.), US is the key imaging method in the diagnosis of MHs.