The sternalis muscle is an anatomic variation well known to anatomists, but relatively unknown to clinicians and surgeons. It is localized superficially to the pectoralis major and can cause a diagnostic dilemma during breast surgery, mammography, and computed tomography and magnetic resonance imaging scans, as its appearance mimics tumor pathology of the region. We studied the presence of longitudinally placed muscles in the anterior thoracic wall in 45 cadavers (90 hemithoraces). In an 83-year-old white male, a rare case of crossed-type sternalis was detected on the left side. The muscle originated from the sternal head of the right sternocleidomastoid, crossed into the opposite parasternal half, and split into 2 tendons and 2 muscle bellies that inserted into the left subcostal arch region. This variant was not included in the available sternalis classifications, and an update is suggested. The muscle is of utmost importance and diagnostic value in routine mammogram screening. Moreover, it is of great value for the plastic surgeon, because identification of the variant can aid the differential diagnosis among other regional lesions. Likewise, its superficial location makes it an ideal candidate for utilization as a muscular flap in plastic reconstruction of the head and neck region.