We describe a case of a 23-year-old man who presented with posterior instability of the right shoulder following trauma. Focal neuropathy of the axillary nerve branch to the teres minor muscle was diagnosed electrophysiologically. A shoulder magnetic resonance image revealed fatty atrophy of the teres minor muscle with sparing of the deltoid, supraspinatus, infraspinatus, and subscapularis muscles. To our knowledge, this is the first case of selective injury of the branch of the axillary nerve to the teres minor muscle to be described in Korea.