None: Syndrome of inappropriate antidiuretic hormone release (SIADH) can sometimes be caused by an adverse effect of certain psychotropic drugs. However, suvorexant has never been reported to cause SIADH. A 77-year-old man with type II diabetes was admitted to the Jichi Medical University Hospital for the treatment of major depression. During the treatment, he was prescribed suvorexant for insomnia. Twelve days after the initiation of suvorexant, he developed hyponatremia, which met the diagnostic criteria of SIADH. We suspected the hyponatremia to be an adverse drug effect of suvorexant because no other cause for SIADH was detected. Accordingly, suvorexant was discontinued 15 days after the onset of SIADH, and hyponatremia improved in 6 days. Although suvorexant has fewer adverse drug reactions and is considered relatively safe, clinicians should be aware of the possibility of SIADH induced by suvorexant.