Background Obesity, insulin resistance and type 2 diabetes mellitus (T2DM) have been associated with endometrial cancer (EC). In this systematic review and meta-analysis we evaluated the effect of metformin on clinical outcomes in patients with EC and insulin resistance or T2DM. Methods Four research databases were searched for original articles published in all languages up to 30 October 2016. Outcomes of interest were overall mortality (OM), cancer-specific mortality, disease progression, and metastases. We performed a random effect meta-analysis of adjusted effects expressed as hazard ratios (HR); heterogeneity among studies was described with the I2 statistic. Results Of the 290 retrieved citations, 6 retrospective cohort studies in women with EC (n = 4723) met the inclusion criteria, and 8.9% to 23.8% were treated with metformin; OM data was available from 5 studies. In 4 studies of EC patients (n = 4132), metformin use was associated with a significant reduction in OM in comparison with not using metformin (adjusted HR [aHR] 0.64, 95% CI 0.45–0.89, p = 0.009). In three studies evaluating patients with EC and T2DM (n = 2637), metformin use was associated with a significant reduction in OM (aHR 0.50, 95%CI 0.34–0.74, p = 0.0006). There was low to moderate heterogeneity of adjusted effects across studies. There was no information about the effect of metformin on cancer-specific mortality, disease progression, or metastases. Conclusions Metformin treatment is associated with a significant reduction in OM irrespective of diabetes status in patients with EC. The survival benefit suggests that diabetes screening and maintenance of good glycemic control may improve outcomes in EC.