An 86-year-old female was found unconscious the day after taking a prescribed tablet containing a combination of tramadol and acetaminophen. At admission to the hospital, marked hypoglycemia (blood glucose: 4 mg/dL) was confirmed, but serum insulin and C-peptide were within the normal range, which suggested that neither endogenous hyperinsulinemia nor exogenous insulin administration was responsible for the hypoglycemia. Despite resuscitation efforts, the woman subsequently died. At autopsy, there was renal disorder, but any pathological abnormalities that could have caused hypoglycemia were not observed. Blood tramadol and acetaminophen were in the therapeutic range. We speculate that the cause of fatal hypoglycemia was tramadol intake at the therapeutic dose. Older age and renal insufficiency are factors that could have potentially caused the fatal hypoglycemia in this case despite tramadol having been taken at a therapeutic dose. This is the first case report of fatal hypoglycemia following ingestion of a therapeutic dose of tramadol.