Objective: DPP-4 inhibitor and α-glucosidase inhibitor combination therapy represents an attractive treatment option by inhibiting the rapid elevation of postprandial blood glucose level without excessive insulin secretion, and potentiation of active GLP-1 secretion. We evaluated the efficacy and safety of adding acarbose in Korean patients with type 2 diabetes showing inadequate response to metformin and sitagliptin combination therapy. Methods: This study was conducted as a multi-center, double-blind, randomized, placebo-controlled study in Korea. Subjects with type 2 diabetes who had been taking metformin (≥ 1,000 mg/day) and sitagliptin were eligible for this study. Participants were assigned to metformin, sitagliptin and acarbose placebo (Met+Sita) group, metformin, stiagliptin and acarbose (Met + Sita + Acarb) group, or metformin placebo, sitagliptin and acarbose (Sita+Acarb) group, at the ratio of 2:2:1. Results: At Week 16, HbA1c decreased by 0.09 ± 0.81 (P = 0.113 vs baseline), 0.44 ± 0.67 (P < 0.001) and increased by 0.84 ± 1.20 (P < 0.001) in Met+Sita (n = 65), Met + Sita + Acarb (n = 66) and Sita+Acarbgroup (n = 34), respectively. The SD of glucose during 75-hr continuous glucose monitoring significantly decreased in Met+Sita+Acarb group at Week 16 compared to the baseline value (P = 0.027). During the mixed meal tolerance test, insulin secretion was similar at Week 16 vs baseline in Met+Sita group (n = 13) and Met+Sita+Acarb group (n = 12). The AUC of glucagon significantly decreased at Week 16 vs baseline (P = 0.008) in Met+Sita+Acarb group, which was not observed in the Met+Sita group. There were no significant differences in the prevalence of the AE, ADR or SAE among three groups. None of the SAE was related to the investigational drugs. Conclusion: A 16-week acarbose add-on therapy to metformin and sitagliptin, effectively lowered HbA1c by 0.44 ± 0.67% and stabilized glucose fluctuation. By acarbose add-on therapy to metformin and sitaglitpin, glucagon secretion was suppressed without excessive insulin secretion during the MTT. Metformin, sitagliptin and acarbose triple combination therapy was generally well-tolerated without significant adverse events.