Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease characterized by the degeneration of alpha motor neurons in the spinal cord, resulting in progressive proximal muscle weakness and paralysis. SMA is classified into types I-IV based on the age at symptom onset or maximum motor function achieved, and its clinical manifestations vary. SMA affects maxillofacial growth because of muscle dysfunction and results in abnormal maxillofacial morphology. In addition, definitive diagnosis is not often made because of the older onset age and symptoms are rarely severe. Therefore, the possibility of undiagnosed SMA in craniofacial surgeries must be considered. This report described a case of an SMA type III recognized after delayed recovery from the neuromuscular blockade in an orthognathic surgery under general anesthesia.