Introduction Insulin glargine 300 U/mL (Gla-300) is a novel glargine formulation which shows slower and more prolonged absorption following subcutaneous administration in comparison to insulin glargine 100 U/mL. In this prospective, observational, single-arm, multicenter, real-world study conducted in Serbia, we evaluated the effectiveness and safety of Gla-300 in patients with type 2 diabetes mellitus (T2DM) previously inadequately controlled with different basal or premix insulin therapy regimes. Methods A total of 350 patients with T2DM were enrolled by 27 physicians, from date of the first patient in (12 December 2017) to the date of last patient completed/last patient out (30 October 2018), from both medical centers and general hospitals. Patients’ observation and data collection were performed at visit 1 (V1), i.e., the inclusion visit (3–6 months after Gla-300 introduction), including collection of retrospective data from the patients’ medical charts at the time of Gla-300 introduction, and at visit 2 (V2) (3–6 months after V1). The primary objective was to assess the change in glycated hemoglobin (HbA1c) level from day of the Gla-300 initiation to the end of the observational period, while the secondary objectives included other effectiveness, as well as safety and other clinically relevant data. Results The mean age of the 350 patients was 63.4 ± 8.4 years and 56.3% were female. The mean duration of diabetes was 13.4 ± 7.4 years, while the mean duration of insulin therapy prior to Gla-300 initiation was 5.3 ± 3.9 years. There was a significant reduction in HbA1c level at each visit compared to the previous visit (8.63 ± 1.52% at baseline prior to Gla-300 initiation, 7.87 ± 1.13% at V1, 7.45 ± 1.05% at V2; p  < 0.01 vs. previous visit) accompanied by significant reduction of all hypoglycemic events ( p  < 0.01). Conclusion Initiation of Gla-300 therapy significantly improved glycemic control and reduced the risk of hypoglycemia in patients with T2DM inadequately controlled with different basal or premix insulin therapy regimes. Funding Sanofi Serbia.