We report an unusual case of hypophosphatemia-related seizure in a child with diabetic ketoacidosis (DKA). A 1-year-old type 1 diabetic boy with hyperglycemia, ketoacidosis, and dehydration was admitted to the pediatric intensive care unit. After having received fluid replacement using isotonic solution with added potassium and continuous intravenous insulin administration according to the protocol for DKA, the patient was conscious, awake, and fed with breast milk. After 20 hours of pediatric intensive care unit stay, he presented 2 tonic-clonic seizures followed by apnea. One hour later, he had cardiorespiratory arrest, requiring cardiovascular support and mechanical ventilation. Serum phosphorus concentration was 1.0 mg/dL, and severe hypophosphatemia was diagnosed. Subsequent to intravenous phosphate replacement, he showed improved neurological and hemodynamic statuses. No other cause of cerebral complication was found. He had no neurologic lesions and was discharged. Although hypophosphatemia is a common complication of DKA treatment, phosphate supplementation has not been routinely recommended in the treatment of DKA. Early recognition and treatment of severe hypophosphatemia in the treatment of DKA are important to reduce the risk of neurological complications.