Background/Aim: Metformin inhibits tumorigenesis in endometrial carcinoma and interferes with the expression of oxidative stress-regulating proteins, such as nuclear factor erythroid 2-related factor 2 (Nrf2) and Kelch-like ECH-associated protein 1 (Keap1). Although manganese superoxide dismutase (MnSOD) is vital for withstanding mitochondrial oxidative stress, it has also been linked with chemoresistance and poorer outcomes in several cancer types. However, data on endometrial cancers are limited. This study aimed to highlight the relationship between mitochondrial redox regulation and endometrial cancer survival in relation to metformin consumption in women with type 2 diabetes mellitus (T2DM). Patients and Methods: Our retrospective hospital-based cohort study included 121 patients diagnosed with endometrial carcinoma and T2DM between 2007 and 2014. Fifty-eight patients were using metformin at the time of diagnosis. Nrf2 and Keap1 expression levels in the tumor samples were assessed immunohistochemically, and MnSOD levels were measured both immunohistochemically and from the serum samples. Results: High MnSOD tissue expression was associated with better overall survival among metformin users in the univariate analysis (p=0.03). When adjusted for histology and stage, high serum MnSOD was associated with better overall survival (HR=0.22, 95%CI=0.07-0.71, p=0.01). No association was found between MnSOD, Nrf2, or Keap1 and overall survival among metformin non-users. Conclusion: Higher expression of MnSOD in patients with endometrial cancer and T2DM is associated with better overall survival if the patient is consuming metformin.