Only a few reports exist in the literature for sonographic assessment of the pectoralis major muscle. Presented is a case of pectoralis major muscle atrophy as a cause of persistent internal rotation weakness diagnosed via ultrasound in a patient with multiple previous surgeries and contraindication to magnetic resonance imaging because of a shoulder implant. This patient's physical examination suggested an abnormal contour of the pectoralis major muscle on contraction, so he was referred for diagnostic ultrasound. The ultrasound was key to guiding the management of this patient because surgical repair of a torn pectoralis major muscle was planned if this was found. No pectoralis major tear or rupture was seen on ultrasound, but there was evidence of pectoralis major muscle atrophy. Accordingly, surgery was avoided and the patient was able to continue with his physical therapy program.