The sternalis muscle is a well-known anatomic variant of the anterior chest wall from anatomists. Nevertheless, surgeons and radiologists are not familiar with this entity. It is the most frequently reported incidental finding during routine anatomic dissection and most common seen during mastectomy. In this case, during a right modified radical mastectomy operation on a 72-year-old female patient, a sternalis muscle was detected on the pectoralis major muscle in the superficial fascia. It was in craniocaudal position and was paralel to the body of sternum, so we protected the muscle. In some cases the sternal muscle is determined intraoperatively and the clinicians have to be familiar with this entity.