Aims:  To investigate the effect of Lactobacillus gasseri BNR17 isolated from human breast milk on blood glucose and body weight in type 2 diabetic animals. Methods and results:  db/db mice were divided into one control group and five sample groups; the sample groups received BNR17 (107, 108, 109 and 1010 CFU) or rosiglitazone (8 mg kg−1) orally twice a day for 12 weeks. BNR17 groups had a dose‐dependent reduction in food, water intake and amount of excrement. Body weight loss was not seen in the BNR17 groups. Fasting and postprandial 2 h blood glucose levels were significantly lower in the BNR17 (1010 CFU) group compared with the control group. HbA1c decreased in the BNR17 group, although it was not statistically significant. During the oral glucose tolerance test, the BNR17 groups exhibited dose‐dependent improvement in glucose sensitivity. Conclusions:  Lactobacillus gasseri BNR17 has a suppressing effect on blood glucose levels and improved diabetic symptoms in db/db mice. Significance and Impact of the Study:  Blood glucose‐lowering lactic acid bacteria are expected to be useful as a therapeutic for treating type 2 diabetes in humans.