Background: The prognostic nutritional index (PNI) reflects systemic inflammation and nutritional status. This study aimed to evaluate the effect of preoperative PNI on postoperative cancer-specific survival in patients with endometrial cancer (EC). Methods: Demographic, laboratory, and clinical data were retrospectively collected from 894 patients who underwent surgical resection of EC. Preoperative PNIs were determined from the serum albumin concentration and total lymphocyte count, which were measured within 1 month before surgery. Patients were classified into high PNI (n = 619) and low PNI (n = 275) groups according to the preoperative PNI cut-off value of 50.6. The stabilized inverse probability of treatment weighting (IPTW) method was used to reduce bias: a weighting cohort divided into high PNI (n = 615.4) and low PNI (n = 272.3) groups. The primary outcome measure was postoperative cancer-specific survival. Results: The postoperative cancer-specific survival rate was higher in the high PNI group than the low PNI group in the unadjusted cohort (93.1% vs. 81.5%; proportion difference [95% confidence interval; 95% CI], 11.6% [6.6–16.6%]; P < 0.001) and in the IPTW- adjusted cohort (91.4% vs. 86.0%; 5.4% [0.8–10.2%]; P = 0.021). In the multivariate Cox proportional hazard regression model in the IPTW-adjusted cohort, high preoperative PNI (hazard ratio [95% CI], 0.60 [0.38–0.96]; P = 0.032) was an independent determinant of postoperative cancer-specific mortality. The multivariate-adjusted restricted cubic spline curve for the Cox regression model showed a significant negative association between preoperative PNI and postoperative cancer-specific mortality (P < 0.001). Conclusion High preoperative PNI was associated with improved postoperative cancerspecific survival in patients undergoing surgery for EC.