The sternalis muscle is an uncommon variation in the anterior chest wall which is located superficial to the pectoralis major, coursing vertically almost parallel to the sternum. It exists unilaterally or bilaterally, as thin muscle strips or a broad band of muscles. We report two cases of skin sparing mastectomy and immediate breast reconstruction in which the sternalis muscle was encountered. This unusual variation might puzzle radiologists to mistake it for a tumor or an abnormal bulging of the pectoralis. Surgeons also must not be surprised to encounter this muscle during surgery of the breast or pectoralis and keep the appropriate dissection plane.