This randomized, prospective study was conducted in 76 subjects to assess whether low-dose (0.5-2 mg/day) glimepiride, in combination therapy with sitagliptin, improves glycemic control in a dose-dependent manner in Japanese patients with type 2 diabetes. Eligible subjects had been treated with glimepiride at doses of 3-6 mg/day for at least 3 months and had a HbA1c level of ≥6.9%. Subjects were randomly assigned to three treatment groups of reduced doses of glimepiride (0.5 mg/day, 1 mg/day, or 2 mg/day) in addition to sitagliptin for 24 weeks. The primary efficacy analysis evaluated the change in HbA1c from baseline to week 24. Secondary efficacy endpoints included the changes in fasting plasma glucose, insulin secretion capacity, and β-cell function. Safety endpoints included hypoglycemia and any adverse event. Despite dose reduction of glimepiride, combination therapy with sitagliptin induced significant improvements in HbA1c levels (-0.8%, p < 0.001). Insulin secretion parameters (CPI, SUIT) also increased significantly. There were no significant differences between groups in changes from baseline HbA1c, insulin secretion capacity, and β-cell function (proinsulin/insulin) at 24 weeks of combination therapy. Multivariate analysis showed that baseline HbA1c was the only predictor for efficacy of combination therapy with sitagliptin and low-dose glimeripide. No changes in body weight were noted and no symptomatic hypoglycemia was documented. These findings indicate that combination therapy with sitagliptin and low-dose glimepiride (0.5 mg/day) is both effective for glycemic control and safe in Japanese patients with type 2 diabetes inadequately controlled with high-dose glimepiride.