The pectoralis major muscle or myocutaneous flap has a nearly 100% success rate in reconstructing chest wall defects. Major adverse sequelae resulting from the use of the pectoralis major muscle or myocutaneous flap are rarely reported in the literature. However, the loss of pectoralis major muscle function caused by the detachment of the muscle from its insertion on the humeral bone is of more and more concern. This is a significant loss for manual laborers when the patient tries to handle tools or control heavy machinery. A case of upper sternal osteomyelitis is reported. After wide debridement with partial excision of the sternum, the second and third ribs, the right pleura, and the lung were exposed. A right unilateral pectoralis major muscle flap was transposed to restore the defect. In addition, to preserve the lateral portion of the muscle and its insertion on the humerus, the origin of the lower sternocostal part of the pectoralis major muscle was transposed to the medial clavicle and residual upper sternum. In this way, not only was the chest wall defect reconstructed but the function of the residual pectoralis major muscle was also preserved. Postoperative follow-up at one year demonstrated no arm weakness, no limitation in shoulder range of motion, and no evidence of atrophy of the transposed pectoralis major muscle. Our experience with this function-preserving pectoralis major muscle flap was encouraging and we suggest it be employed in the reconstruction of the upper anterior chest wall.