Rasmussen encephalitis (RE) is a rare and severe brain disorder that is associated with unilateral hemispheric atrophy and manifests as severe refractory epilepsy, hemiplegia, defects of motor and speech functions, and cognitive impairment. Treatment of RE, especially in adult patients, is extremely challenging. Herein, we report the case of an adult patient with RE who was treated with a functional hemispherectomy and achieved a favorable prognosis. A 29-year-old woman presented with a 24-year history of epileptic seizures. Neurological examination showed hemiplegia, homonymous hemianopsia, and right muscular atrophy. Neuropsychological examination demonstrated cognitive disorders. Serial magnetic resonance imaging (MRI) and computed tomography (CT) scans showed progressive encephalatrophy in the left hemisphere and ventriculomegaly in the left lateral ventricle. The Wada test showed that the right hemisphere was dominant for language. A functional hemispherectomy was performed. Postoperatively, no antiepileptic drugs were administered, and the patient remained seizure-free without aggravation of hemiplegia. Over a 2-year follow-up, cognitive functions improved. In an adult patient with RE, a favorable prognosis was achieved after functional hemispherectomy. The safety and efficacy of functional hemispherectomy in patients with RE is highlighted.