A unilateral four-headed pectoralis major muscle was observed on the left side of an 83 year-old Caucasian male cadaver. The accessory tendon arises from the lateral aspect of the pectoralis major muscle, travels along with the tendon of the long head of the biceps brachii and blends into the capsule of the shoulder joint. This anomaly is rare. A complete or partial absence of pectoralis major muscle is normally reported. Additionally, supernumerary heads of muscles other than pectoralis major have been documented. However, the existence of an accessory tendon to the pectoralis major muscle is unique. The implications of such a finding are discussed.