We report a 75-year-old woman developing serotonin syndrome following minimum doses of sertraline. She showed a depressed mood, insomnia, and general fatigue and was taking sulpiride at 300 mg/day, alprazolam at 1.2 mg/day, zopiclone at 7.5 mg/day, and etizolam at 1 mg/day. As she remained symptomatic, sertraline at 25 mg/day was added. Within 14 hours of starting sertraline, the patient began to experience delirium, impaired coordination, diaphoresis, tremulousness of the upper limbs bilaterally, and agitation. Sertraline was thus discontinued, and all of the above-mentioned symptoms disappeared rapidly. Serotonin syndrome is rarely reported in patients taking sertraline in Japan. To our knowledge,ours is the second case of serotonin syndrome associated with sertraline in Japan. According to Drug in Japan, sertraline must be started at the lowest efficacious dose with slow titration and is contraindicated for patients who are taking pimozide or monoamine oxidase inhibitors（MAOIs）. Also,the coadministration of sertraline with other agents such as lithium, tricyclic antidepressants,and triptans necessitates the close observation of symptoms and signs. However, our case didn't take any of these combinations, and she was administered 25 mg/day, the lowest efficacious dose. This report emphasizes that caution is needed when prescribing sertraline to elderly patients and on its coadministration.