Background: Gestational diabetes mellitus (GDM) is a common pregnancy complication associated with adverse maternal and fetal outcomes. Insulin is the standard treatment; however, metformin is increasingly used due to oral route and improved compliance. Objective: To compare the efficacy of metformin plus insulin with insulin alone in achieving glycemic control among pregnant women with GDM. Methods: This randomized controlled trial was conducted at Capital Development Authority Hospital, Islamabad. A total of 62 pregnant women with GDM not controlled with diet were randomly allocated into two groups (n = 31 each). Group A received insulin alone, while Group B received metformin in addition to insulin. Glycemic control was assessed using fasting blood glucose (<95 mg/dl) and 2-hour postprandial glucose (<120 mg/dl). Data were analyzed using SPSS. Results: Baseline characteristics were comparable between groups. A significantly higher proportion of patients achieved glycemic control in the metformin plus insulin group compared to the insulin-alone group (83.9% vs 41.9%, p = 0.001). The combination group had lower follow-up fasting glucose (90.32 ± 3.93 vs 94.63 ± 4.18 mg/dl, p < 0.001), lower postprandial glucose (113.94 ± 4.34 vs 119.71 ± 7.16 mg/dl, p < 0.001), and shorter time to control (7.16 ± 2.27 vs 25.55 ± 10.59 days, p < 0.001). Conclusion: Metformin combined with insulin is more effective than insulin alone in achieving glycemic control in GDM and reduces the time required to achieve target glucose levels.