During the routine dissection studies on the right side of a 56-year-old female cadaver we encountered co-existence of the pectoralis quartus and pectoralis intermedius muscles. The pectoralis quartus originated from the costochondral junction of the fifth and sixth ribs, and then extended laterally under the border of pectoralis major muscle, but it was entirely separate from it. The pectoralis quartus formed a long flat band with an average width of 1.5 cm. It then inserted as an aponeurosis to the both of lateral lip of the intertubercular groove of the humerus and tendon of the short head of the biceps brachii muscle. Furthermore, the pectoralis intermedius muscle was a fleshy slip between the pectoralis minor and pectoralis quartus muscles and arose from the third and fourth ribs. It then united to the tendon of the short head of the biceps brachii muscle two cm below the coracoid process.