During routine dissection practice a supernumerary muscle was found on the right side infraclavicular region of a male cadaver. This muscle was arising from the superior surface of the first rib and its coastal cartilage and inserted to a thick ligament that extended from the medial end of the suprascapular notch to the capsule of the acromioclavicular joint. This accessory muscle slip was innervated by a branch from the nerve to subclavius. According to its location and innervation the aberrant muscle was considered to be the subclavius posticus. The anatomic relationships of the muscle make it clinically significant