IntroductionOnce-weekly (OW) glucagon-like peptide 1 receptor agonist (GLP-1RA) semaglutide has been shown to have a more potent glycated hemoglobin (HbA1c)-lowering effect than other oral hypoglycemic agents and existing GLP-1RAs in global randomized controlled trials. The study aim was to evaluate the safety and effectiveness of OW semaglutide in Japanese patients with type 2 diabetes mellitus (T2DM) in a real-world clinical setting and identify pre- and post-treatment predictors of good response.MethodsWe investigated the change in HbA1c, percentage of patients achieving < 7% HbA1c, and factors contributing to the effect 6 months after OW semaglutide use in Japanese patients with T2DM. We also examined differences in effectiveness between patients with different backgrounds.ResultsAt baseline, the 77 patients had a mean baseline HbA1c of 8.1% ± 1.23%, 74% of the patients were injecting another GLP-1RA, and 42.9% of the patients were being treated with insulin. HbA1c decreased by 0.89% and by 0.66% in the other GLP-1RA users. The rate of achievement of < 7% HbA1c increased from 21% to 43%. There were no differences in effect by age, sex, or body mass index. Higher baseline HbA1c and shorter duration of diabetes were associated with greater HbA1c reduction. OW semaglutide was tolerable for the majority of our study population.ConclusionThis study provided real-world evidence showing that OW semaglutide significantly reduced HbA1c in Japanese patients with T2DM who had inadequate HbA1c control.Supplementary InformationThe online version contains supplementary material available at 10.1007/s13300-022-01313-0.