Hyponatremia is an emerging issue in cancer patients. However, few studies have explored the prognostic value of preoperative hyponatremia in epithelial ovarian cancer (EOC) patients. We conducted a retrospective study of women diagnosed with EOC at Shengjing Hospital from 2011 to 2015. Preoperative serum sodium levels were measured and classified as normal (136-145 mmol/L) or hyponatremic (≤135 mmol/L) according to the average/most recent measurements of this biomarker. Multivariable Cox proportional hazard regression models were applied to determine the adjusted hazard ratios (HRs) with 95% confidence intervals (CIs) for the association of preoperative hyponatremia with progression-free survival (PFS) and overall survival (OS). We identified 607 EOC patients meeting the inclusion criteria with a median age of 51 years (inter-quartile range: 47-59 years). The median follow-up duration was 3.1 years (inter-quartile range: 2.2-4.2). The recurrence and mortality rates were 56.8% (345/607) and 38.4% (233/607), respectively. Among the patients, 81 (13.3%) were observed as hyponatremic. Preoperative hyponatremia was associated with poorer PFS (HR=1.51; 95%CI=1.07-2.15) as well as OS (HR=1.47; 95%CI=1.03-2.11) after adjusting for potential confounders. Notably, results were in line with the main findings only when using the most recent serum sodium levels before treatment, with corresponding HRs of 1.58 (95%CI=1.12-2.23) for PFS and 1.45 (95%CI=1.02-2.07) for OS. Preoperative hyponatremia is an independent prognostic factor of EOC. Further studies are warranted to confirm our findings as well as to determine whether correction of preoperative hyponatremia may alter clinical outcomes in these patients.