Diosmin is one of the flavonoids contained in citrus and has been demonstrated to improve glucose metabolism in diabetic disorders. However, the mechanism(s) of diosmin in glucose regulation remain obscure. Therefore, we investigated the potential mechanism(s) for the antihyperglycaemic action of diosmin in streptozotocin‐induced diabetic rats (STZ‐diabetic rats). Diosmin lowered hyperglycaemia in a dose‐dependent manner in STZ‐diabetic rats. This action was inhibited by naloxone at a dose sufficient to block opioid receptors. Additionally, we determined the changes in plasma β‐endorphin‐like immunoreactivity (BER) using enzyme‐linked immunosorbent assay (ELISA). Diosmin also increased BER dose‐dependently in the same manner. Repeated treatment of STZ‐diabetic rats with diosmin for 1 week resulted in an increase in the expression of the glucose transporter subtype 4 (GLUT 4) in the soleus muscle and a reduction in the expression of phosphoenolpyruvate carboxykinase (PEPCK) in the liver. These effects were also inhibited by naloxone at a dose sufficient to block opioid receptors. Bilateral adrenalectomy in STZ‐diabetic rats eliminated the actions of diosmin, including both the reduction in hyperglycemia and the elevation of plasma BER. In conclusion, our results suggest that diosmin may act on the adrenal glands to enhance the secretion of β‐endorphin, which can stimulate the opioid receptors to attenuate hepatic gluconeogenesis and increase glucose uptake in soleus muscle, resulting in reduced hyperglycemia in STZ‐diabetic rats.