Patients with schizophrenia often experience problems associated with ordinary exercises of life due to their mental symptoms. Those experiencing problems related to feeding behavior, in particular, are considered to be susceptible to developing Wernicke encephalopathy due to a deficiency in thiamine, the physiological stores of which are limited; however, there are few reported cases, and most of them were accompanied by the classical triad of signs. We report our experience with asymptomatic thiamine deficiency (TD) in a schizophrenia patient. A 73-year-old female was receiving medication for schizophrenia as an outpatient. No symptoms such as hallucinations or delusions were observed, the patient had a sociable personality and was able to function at a level where she could live alone. Although there were no active complaints about eating by the patient, we investigated the situation due to reports of TD in schizophrenia patients. As results revealed a significant decrease in whole blood thiamine to 19 ng/mL (reference range: 24-66 ng/mL), we administered a large dose of thiamine. No changes were observed in psychosomatic symptoms before and after administration. Patients with schizophrenia experience problems that may lead to TD, such as dietary imbalances and disturbed feeding habits. Therefore, even if patients with schizophrenia do not actively complain about their feeding behavior, it may be necessary to take medical measures such as blood sampling in consideration of the potential for developing TD.