Ultrasound-guided cervical medial branch block (CMBB) is commonly performed to diagnose and treat head, neck, and shoulder pain. However, its use at the C7 level has been shown to be less accurate than at other levels, which may increase the chance of injury owing to the imprecision of needle site provided by the ultrasound guide. We report the first case of iatrogenic spinal cord injury from an ultrasound-guided C7 CMBB. The patient, upon receiving this procedure, had fainted shortly after experiencing an electrical sensation that ran from the neck to the toe. The patient complained of weakness and tingling sensation in the left upper extremity. Cervical magnetic resonance imaging revealed a hematoma in the cervical spinal cord, and an electrophysiological study, which was performed at 3 weeks after the incident, revealed an injury at the left C3-T2 anterior horn. After 2 months of rehabilitation, the patient showed moderate improvement in the strength of the left proximal upper extremity; however, there was no improvement in the strength of the left distal upper extremity. Therefore, we recommend caution when performing ultrasound-guided CMBB at the C7 level, as the guide particularly at this level is relatively inaccurate, posing a risk of spinal cord injury.