Hyperbaric oxygen (HBO) is widely accepted as a treatment for air or gas embolism, carbon monoxide (CO) poisoning, clostridial myonecrosis, crush injuries, and thermal burns. To the best of our knowledge, after HBO therapy, atrial fibrillation (AF) has not been reported in the literature yet. We herein describe a case of AF that occurred in a woman who had taken HBO therapy for CO poisoning. The 78-year-old female patient was admitted to the emergency department with less of consciousness. Carboxyhemoglobin value at arterial blood gases was found to be 42.6% and was thought to be CO poisoning. Electrocardiography (ECG) was normal sinus rhythm. Hyperbaric oxygen therapy was planned. After the HBO therapy, ECG showed AF. Her rhythm returned to the normal sinus rhythm after amiodarone treatment. Physicians should keep in mind that HBO treatment could contribute to AF, and all patients' ECG should be monitored before and after HBO therapy.