Introduction: The brachial plexus is the largest and most complex neural plexus in the body located in the neck and axilla. So far, there have been many reports of various variations in the brachial plexus that are of clinical significance. In the present study, while dissecting the upper limb of a 54-year-old man in the dissection room of Shahid Sadoughi University of Medical Sciences in Yazd, a rare variation was observed in the lateral cord branches of the plexus. In this case, the medial pectoral nerve, which normally separates from the medial cord, originated from the lateral cord. Awareness of this variation is important for anatomists, surgeons, anesthesiologists and radiologists to interpret the graphs, etc., and can help to reduce clinical complications during surgery and better interpret and diagnose the graphs.