Objective : To compare the efficacy and safety of ipragliflozin versus placebo as add-on therapy to metformin and sitagliptin in Korean patients with type 2 diabetes mellitus (T2DM). Methods : This double-blind, parallel-group, placebo-controlled, phase III study was conducted at 22 sites in Korea in 2015-2017. Outpatients (aged 19~74 years) with T2DM inadequately controlled with metformin and sitagliptin were recruited. Following 2 weeks of placebo run-in patients were randomized to receive either 50mg ipragliflozin or placebo once daily for 24 weeks in addition to metformin and sitagfiptin. The primary endpoint was change in glycosylated hemoglobin (HbA1c) from baseline to end of treatment (EOT). Results : Of 143 randomized patients, 139 were included in efficacy analysis(ipragliflozin: 73, placebo: 66). Baseline mean (SD) HbA1c levels were 7.90% (0.69) for ipragliflozin and 7.92% (0.79) for placebo. The corresponding mean (SD) change in HbA1c from baseline to EOT were -0.79% (0.59) and 0.03% (0.84), respectively with Compared with placebo, lasting plasma glucose, fasting serum insulin, body weight, and homeostatic model assessment for insulin resistance decreased significantly at EOT with ipragliflozin, with between-group difference of -29.55 mg/dl, -1.50 μU/mL, -1.72kg, and -0.99, respectively(p<0.05 for all). More ipragliflozin-treated than placebo-treated patients achieved HbA1c targets of <7.0% (44.4% vs. 12.1%) and <6.5% (12.5% vs. 1.5%) at EOT(p<0.05 for both). Adverse event rates were similar between groups (ipragliflozin: 51.4% vs. placebo: 50.0%). There were no reports of hypoglycemia, volume depletion, genital infection, and ketoacidosis for ipragliflozin. Incidences of urinary tract infection and plyuria/pollakiuria were low (ipragliflozin: 2.7% and 1.4%, respectively vs. placebo: 1.5% for both). Conclusion: Addition of ipragliflozin to metformin and sitagliptin therapy demonstrated significant improvements in glycemic parameters and a good safety profile in Korean patients with inadequately controlled T2DM.