Dienogest is a fourth-generation progestin that is used for the treatment of endometriosis. We report a case of premenstrual mood changes in a patient with schizophrenia who was unresponsive to conventional treatment but successfully managed with dienogest. A 37-year-old Japanese woman with schizophrenia was referred to our hospital and diagnosed with premenstrual exacerbation of schizophrenia or coexisting premenstrual dysphoric disorder with schizophrenia. She had already taken maximal doses of selective serotonin reuptake inhibitors and combined oral contraceptives produced intolerable side effects. Gonadotropin-releasing hormone agonist treatment was effective but was not suitable for long-term use. Dienogest was initiated to treat pelvic endometriosis and produced subsequent improvements in mental status. The patient was able to return to work and did not indicate any adverse effects. This case suggests that dienogest may be useful for managing premenstrual mood changes in patients with schizophrenia, that it can be safely administered over long periods of time.