The axillary arch of Langer (Axillopectoral muscle) is the most common anatomical variant of the axillary musculature. The incidence of the variant is about 7% or less in the population and despite the reported frequency, its presence has been rarely reported. A 33-yr-old woman visited our institution due to the presence of a right breast mass and was diagnosed with right breast cancer by an ultrasound guided core biopsy. Previously, the patient had had no complaints of sensory and motor dysfunction of the right arm. The patient underwent a modified radical mastectomy and we identified an abnormal muscle that originated from the latissimus dorsi, and was inserted in the trilaminar tendon of the pectoralis major during axillary lymph node dissection. The muscle interfered the level I area due to its longitudinally oblique direction. We separated this muscle, and carefully dissected the medial axillary group and lateral axillary group. After follow-up for two years, there was no evidence of axillary recurrence, lymphedema or any limitation of motion of the right arm. We discuss the definition, clinical complications and clinical importance of the axillary arch of Langer. We report here the first case of axillary arch of Langer that was identified during surgery in Korea.