During lymphadenectomy in the left axilla of a 38-year-old woman with a 1.4 cm invasive ductal breast carcinoma an accessory muscle was found. Due to the presence of the anomalous muscle the lymphadenectomy was carried out with difficulty through a limited field. Based on its anatomical characteristics, the supernumerary muscle was recognized as the pectoralis quartus. To our knowledge this is the first report of a pectoralis quartus muscle as a surgical finding. The surgeon should be aware of the possible presence of this anomaly as well as its anatomical characteristics in order to avoid any complications.