Osmotic demyelination syndrome is a pathological condition that leads to electrolyte imbalances and rapid correction, resulting in pseudobulbar palsy, quadriplegia, and altered consciousness. Approximately 33-55％ of affected patients experience residual functional impairment. Herein, we describe a case of a patient with osmotic demyelination syndrome who developed locked-in syndrome during the disease course, underwent rehabilitation treatment, and achieved complete remission without sequelae.The patient was a 47-year-old man who was admitted to hospital A owing to weakness in the lower extremities and dysarthria. He had severe hyponatremia and received sodium correction. However, on hospital day 9, dysarthria redeveloped and involuntary finger movements were noted. Osmotic demyelination syndrome was suspected based on the findings of magnetic resonance imaging of the head and clinical course, leading to his transfer to hospital B. Steroid pulse and rehabilitation therapies were initiated at hospital B. By the 19th day of symptom onset, his limb and facial muscle paralysis progressed, leading to locked-in syndrome. Thereafter, the patient was transferred to hospital C, where he received physiotherapy, occupational therapy, and eating training, markedly improving his physical functions. He was discharged from hospital C, 4 months after the symptom onset, with limited range of motion of the fingers and weakness of the extremities and continued to receive outpatient rehabilitation treatment. His symptoms improved further, and 1 year after the onset of symptoms, he returned to work without any sequelae.