G protein-coupled receptor 119 (GPR119) is a promising target for the treatment of type 2 diabetes mellitus (T2DM), as both insulin and glucagon-like peptide-1 secretion can be promoted with a single drug. We compared the efficacy and safety of the GPR119 agonist DS-8500a with placebo and sitagliptin 50 mg in Japanese patients with T2DM. This randomized, double-blind, parallel-group comparison study was conducted in Japan (trial registration NCT02628392, JapicCTI-153068). Eligible patients aged ≥ 20 years with T2DM and hemoglobin A1c (HbA1c) ≥ 7.0% and < 10.0% were randomized to receive placebo, DS-8500a (25, 50, or 75 mg), or sitagliptin 50 mg once daily for 12 weeks. The primary efficacy endpoint was change in HbA1c from baseline to week 12. Secondary endpoints included change in fasting plasma glucose (FPG), glucose AUC0-3h during a meal tolerance test, 2-hour postprandial glucose (2hr-PPG), and changes in lipid parameters (total, low-density lipoprotein (LDL-) and high-density lipoprotein (HDL-) cholesterol, and triglycerides) at week 12. Safety endpoints included adverse events, hypoglycemia, and clinical/laboratory variables. DS-8500a demonstrated dose-dependent HbA1c lowering compared with placebo at week 12: change from baseline - 0.23% (p = 0.0173), - 0.37% (p = 0.0001), and - 0.44% (p < 0.0001) in the 25-mg, 50-mg, and 75-mg groups, respectively. At 50- and 75-mg doses, DS-8500a significantly lowered FPG, glucose AUC0-3h, and 2hr-PPG compared with placebo. The glucose-lowering effect was maintained up to 12 weeks. DS-8500a did not lower any of the above parameters to a greater extent than sitagliptin. Compared with placebo and sitagliptin, DS-8500a 50 and 75 mg significantly reduced total cholesterol, LDL-cholesterol, and triglycerides, and significantly increased HDL-cholesterol. All DS-8500a doses were well tolerated. Two cases of clinically relevant drug-related hypoglycemia occurred in the DS-8500a 50-mg group. DS-8500a was well tolerated and demonstrated significant glucose-lowering effects and favorable changes in lipid profiles up to 12 weeks in Japanese patients with T2DM. Daiichi Sankyo Co. Ltd.