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A rare muscle sternoclavicularis was found in a large triangular gap between the sternocostal and clavicular heads of Pectoralis Major muscle on the right side during routine cadaveric dissection. Sternoclavicularis was seen to arise from the anterior surface of manubrium sterni and the capsule of sternoclavicular joint and was inserted on the anterior surface of middle one third of the clavicle. It was supplied by the lateral pectoral nerve. Sternoclavicularis muscle may help in stabilizing the clavicle and may partially fill the triangular deficit in the origin of the Pectoralis Major. This variation may be of particular interest to plastic surgeons, orthopaedic surgeons, radiologists and neurologists. It may mimic a tumour at this site.