A 42-year-old woman with short-term memory loss visited Gangnam Severance Hospital, and her chest X-ray and computed tomography revealed a right anterior mediastinal mass. On hospital day two, she suddenly presented personality changes and a drowsy mental status, so she required ventilator care in the intensive care unit. She underwent thymectomy, and was pathologically diagnosed with thymoma, type B1. Her mental status eventually recovered by postoperative day 90. Paraneoplastic encephalopathy associated with thymoma is very rare, and symptoms can be improved by thymectomy. We report a case of paraneoplastic encephalopathy associated with a thymoma.