The lateral pectoral nerve (LPN) is commonly injured along with the brachial plexus, but its isolated lesions are rare. Here, we present a case of an isolated LPN lesion confused with cervical radiculopathy. A 41-year-old man was admitted to our clinic because of weakness in his right arm. Previous magnetic resonance imaging (MRI) examination revealed right posterolateral protrusion at the C6-7 level. At the initial assessment, atrophy of the right pectoralis major muscle was evident, and mild weakness of the right shoulder adductor, internal rotator, and flexor muscles was observed. Therefore, electrodiagnostic evaluation was performed, and a diagnosis of isolated LPN injury was made. Nerve injury was thought to have been caused by weightlifting exercises and traction injury. Lateral pectoral nerve injury can mimic cervical radiculopathy, and MRI examination alone may lead to misdiagnosis. Repeated physical examinations during the evaluation and treatment phase will identify the muscle atrophy that occurs 1 or more months after the injury.