Evidence indicates that type 2 diabetes may stimulate the initiation and progression of several types of cancer. Metformin, a drug most commonly used to treat type 2 diabetes, may inhibit cancer cell growth and reduce the risk of cancer. However, evidence of the antitumor effects of metformin on ovarian cancer is still limited.In this study, we retrospectively examined the effects of metformin on ovarian cancer patients with diabetes at our institution.We identified 568 consecutive patients who were newly diagnosed with ovarian cancer and treated between January 2011 and March 2014. Patients with International Federation of Gynecology and Obstetrics (FIGO) stage I to IV epithelial ovarian, fallopian, or peritoneal cancer were included. Patients with type 1 diabetes, incomplete records (including medication records) and any other cancer before their ovarian cancer diagnosis, as well as those diagnosed with diabetes more than 6 months after their ovarian cancer diagnosis, were excluded. Out of 568 patients, 48 (8.5%) patients with type 2 diabetes continuously used metformin, 34 (5.9%) patients with type 2 diabetes did not take metformin, 22 (3.9%) patients with type 2 diabetes discontinued metformin, and 464 (81.7%) ovarian cancer patients were nondiabetic controls. Longer progression-free survival (PFS) and overall survival (OS) were observed in ovarian cancer patients with diabetes who were taking metformin than in diabetic patients not taking metformin, diabetic patients who discontinued metformin, and nondiabetic ovarian cancer patients (P = .001). After adjusting for possible confounders, metformin use was associated with a lower risk for disease relapse [hazard ratio (HR) = 0.34; 95% confidence interval (CI): 0.27-0.67; P < .01] and disease-related death (HR = 0.29; 95% CI: 0.13-0.58, P = .03) among ovarian cancer patients with diabetes.Metformin use may decrease the risk for disease recurrence and death in patients with ovarian cancer, but the drug treatment must be continuous.