Guillain-Barré syndrome is a rare autoimmune condition characterized by ascending motor weakness of the extremities that can ascend to the diaphragm, causing substantial morbidity and mortality. This case report describes a 57-year-old man who exhibited characteristics of Guillain-Barré syndrome 9 days after undergoing lumbar fusion at L3-S1. The diagnosis was based on the patient's ascending motor weakness and areflexia and was confirmed with electromyography. The patient progressed to respiratory failure, requiring mechanical ventilation. He regained motor function and ambulation within 6 months. Although the syndrome typically manifests initially as ascending paralysis, this patient's initial symptom was new-onset atrial fibrillation, a sign of autonomic dysfunction. Because it can cause paralysis and respiratory failure, Guillain-Barré syndrome should be included in the differential diagnosis whenever motor weakness is observed after lumbar surgery. The timing of symptoms, imaging results, and the development of atypical symptoms can help distinguish this rare possibility from other postoperative spinal complications.