In hand reconstructive surgery the palmaris longus muscle is one of the most utilized donor site for tendon reconstruction procedures. However, its anatomic position is variable and anatomic variations may be responsible for median nerve compression. We report the case of a 40-year-old, right-handed woman, who presented with numbness and paresthesias in the palm and in the flexor aspect of the first, second, and third fingers of her right hand for the preceding 5 months, coinciding with increase of office work (typing). The clinical examination and radiological investigations (ultrasound and magnetic resonance) revealed a subcutaneous mass (15 mm x 2.3 mm x 6 cm), with a lenticular shape and definite edges at the level of the volar aspect of the distal third of the forearm. The fine-needle aspiration biopsy revealed the presence of striated muscle fibers. During surgery, a muscle belly was found in the epifascial plane. This muscle originated from subcutaneous septa in the middle forearm and inserted on to the superficial palmar aponeurosis with fine short tendon fibers. Exposure of the antebrachial fascia did not reveal any area of weakness or muscle herniation. The palmaris longus tendon, flexor digitorum superficialis tendons, and flexor carpi radialis tendon showed usual topography under the antebrachial fascia. The accessory muscle was excised and histology revealed unremarkable striated muscle fibers, limited by a thin connective sheath. The presence of an accessory palmaris longus (APL) located in the epifascial plane could be ascribed to an unusual migration of myoblasts during the morphogenesis. Although extremely rare, APL is worth bearing in mind as a possible cause of median nerve compression and etiology of a volar mass in the distal forearm.