Objective: Limited real-world evidence is available on the effectiveness of liraglutide compared with sitagliptin as an add-on therapy to metformin in clinical practice for adult patients with Type 2 diabetes mellitus (T2DM). The purpose of this study was to compare clinical outcomes 6 months after initiating treatment with liraglutide or sitagliptin for patients uncontrolled on metformin monotherapy in the United States (US). Methods: We used the General Electric Centricity electronic medical records database to analyze the effectiveness of liraglutide and sitagliptin in adult (≥18 years) patients with T2DM who started either drug between January 1, 2010 and January 31, 2013 (index period) as an add-on to metformin monotherapy. Changes in A1C, body weight, and the proportion of patients reaching ADA A1C target of <7.0% 6 months after starting treatment with liraglutide or sitagliptin, adjusted for differences in demographic and baseline clinical characteristics, were evaluated. Results: 395 patients treated with liraglutide (mean age: 52.9 years, female: 57.5%) and 1,896 patients treated with sitagliptin (mean age: 58.0 years, female: 51.1%) were identified during the index period. After adjusting for baseline factors, patients treated with liraglutide experienced greater reductions in A1C (-1.18% vs. -0.94%, p<0.001) and body weight (-3.0 kg vs. -1.6 kg [6.6 lbs vs. -3.4 lbs], p<0.001) at 6 months from baseline than patients treated with sitagliptin. Significantly more patients treated with liraglutide met the A1C <7.0% target after 6 months follow-up (49.4% vs. 40.0%, p=0.001). Conclusions: As in clinical trials, this real-world study found that among adult patients with T2DM uncontrolled on metformin monotherapy liraglutide was associated with significantly greater reductions in A1C and body weight and improved glycemic goal attainment compared to sitagliptin.