Obesity and gestational diabetes mellitus (GDM) have an independent negative impact in pregnancy outcomes. Excessive gestational weight gain (GWG) represents an additional high-risk condition for adverse outcomes. The aims of this study were to evaluate the potential effect of metformin in GWG in overweight or obese women with GDM, to report our experience and to assess metformin's safety in this population.Retrospective observational cohort study involving pregnant women with GDM and pregestational overweight or obesity. Demographic, anthropometric, glycemic control data, obstetric, fetal and neonatal outcomes were evaluated. The sample was divided into two groups according to metformin treatment. A propensity score-matched analysis was performed using age, initial body mass index (BMI), trimester at GDM diagnosis and previous history of GDM or macrosomia as covariates.Of the 457 enrolled in the study, 177 (38.7%) were treated with metformin. Two groups of 130 well matched patients were balanced regarding baseline characteristics. Women in metformin group had significantly less excessive GWG (29.23% vs. 42.31%, OR 0.56, 95% CI 0.34-0.94, p = 0.028) and more adequate GWG (36.92% vs. 23.08%, OR 1.95, 95% CI 1.14-3.35, p = 0.015). No significant differences were found between both groups regarding glycemic control, rate of insulinization, and obstetric, fetal, and neonatal outcomes.This study highlights metformin as an important and safe tool to prevent excessive GWG and promote adequate GWG in overweight or obese women with GDM, regardless of age, BMI, timing of GDM diagnosis, previous history of GDM or macrosomia.