OBJECTIVE Atypical antipsychotic-induced hyponatremia has been reported in patients with psychiatric disorders. To date, hyponatremia due to lurasidone, an atypical antipsychotic approved for the treatment of schizophrenia and bipolar depression, has never been reported. METHODS A female patient with bipolar depression and a history of subdural hematoma experienced a rapid onset of hyponatremia after the initiation of low-dose lurasidone. The hyponatremia worsened after the dose of lurasidone was increased and resolved only after lurasidone was ceased. According to the Naranjo Adverse Drug Reaction Probability Scale, this case report scores 6 as a possible drug reaction between lurasidone and hyponatremia. RESULTS Based on laboratory results, the syndrome of inappropriate antidiuretic hormone secretion and thyroid or adrenal dysfunction as differential diagnoses were excluded. Lurasidone-induced polydipsia complicated by hyponatremia was confirmed. Lurasidone was subsequently discontinued. The sodium level returned to normal within 1 week without any sodium supplementation. CONCLUSIONS This case report highlighted that low-dose lurasidone may induce polydipsia complicated by hyponatremia. Physicians should be aware of the adverse reactions of hyponatremia associated with lurasidone, particularly in patients with a history of intracranial hemorrhage.