A hyperosmolar hyperglycemic state (HHS) is a life-threatening complication rarely seen in children and adolescents with type 1 diabetes mellitus (T1DM). However, early diagnosis and proper treatment are vital to reduce the high morbidity and mortality rates associated with HHS. We describe a male patient who presented with polydipsia, polyuria, and a drowsy mental status. His initial biochemistry results demonstrated severe hyperglycemia (1,456 mg/dL), hyperosmolarity of 359 mOsm/kg (effective osmolarity, 323 mOsm/kg), and mild acidosis (venous pH, 7.327). The patient was diagnosed with HHS and T1DM based on the presence of hyperosmolarity, hyperglycemia, and positivity for antiglutamic acid antibodies. Intensive intravenous fluid and regular insulin (0.025 units/kg/hr) were administered. After hydration and insulin treatment, the patient's mental status and serum glucose and sodium levels improved, and no neurological complications were observed. In summary, most cases of HHS are observed in adult patients with type 2 diabetes. However, occurrences in children and adolescents with T1DM have also been reported. Therefore, HHS should be considered in the differential diagnosis of hyperglycemic emergencies.