Purpose: To report the case of a 14-year-old female patient with one-and-a-half syndrome subsequently diagnosed with multiple sclerosis involving the pons.Case summary: A 14-year-old girl without any underlying disease presented with difficulty focusing and mild headache for 5 days. The patient showed conjugate gaze palsy to the left, limited adduction in the left eye, and abducting nystagmus in the right eye, which indicated one-and-half syndrome. T2 fluid-attenuated inversion recovery brain magnetic resonance imaging revealed multiple punctate hyperintensities in the pontine tegmentum, bilateral cerebral white matter, and cerebellum. Cerebrospinal fluid examination revealed oligoclonal bands and multiple sclerosis was diagnosed. The patient was treated with intravenous steroids and beta-interferon. Seven weeks later, limitation of eyeball movement and nystagmus had resolved completely.Conclusions: At the young age of 14 years, a patient presenting with one-and-a-half syndrome was diagnosed with multiple sclerosis. Children with multiple sclerosis may experience severe physical and cognitive impairments, and brainstem involvement predicts an especially poor prognosis. Early diagnosis and active treatment may help to prevent poor outcomes.