A recent report described that sodium glucose cotransporter (SGLT) 1 and 2 were expressed in α cells, and dapagliflozin which is a specific SGLT2 inhibitor, increased plasma glucagon levels by inhibiting SGLT2 in α cells. However, it is still controversial whether SGLT2 inhibitors increase plasma glucagon levels in vivo. Here, we showed that dapagliflozin, but not canagliflozin which is a SGLT2/low-potency-SGLT1 inhibitor, increased plasma glucagon levels in mice fed a high-fat, high-sucrose diet (HFHSD) and diabetic db/db mice. A glucose clamp study revealed that the plasma glucagon increase associated with dapagliflozin could be explained as a response to acute declines in blood glucose. RT-PCR and whole RNA sequencing revealed that SGLT1 and glucose transporter 1 (GLUT1), but not SGLT2, were expressed in αTC1 cells and mouse islets. We showed that canagliflozin suppressed glucagon secretion by inhibiting SGLT1 in α cells; consequently, plasma glucagon did not increase with canagliflozin, even though blood glucose declined. We also showed that the SGLT1 effect on glucagon secretion depended on sodium/glucose cotransport, but not on metabolic effect of glucose. Islets from HFHSD and db/db mice displayed higher SGLT1 expression and lower GLUT1 expression than the islets from control mice. These expression levels were associated with glucagon secretion levels. Furthermore, SGLT1 inhibitor and siRNA against SGLT1 suppressed glucagon secretion in isolated islets. These data suggested that a novel mechanism regulated glucagon secretion, through SGLT1, in α cells. This finding explained the distinct effects of dapagliflozin and canagliflozin on plasma glucagon levels in mice. Disclosure T. Suga: None. O. Kikuchi: None. M. Kobayashi: None. S. Matsui: None. H.H. Hashimoto: None. T. Sasaki: None. S. Kakizaki: None. M. Yamada: None. T. Kitamura: None.