Many established inflammation- and nutrition-related factors have been investigated as potential independent prognostic factors in various cancers, including the C-reactive protein/albumin ratio (CAR), lymphocyte/monocyte ratio (LMR), modified Glasgow prognostic score (mGPS), body mass index (BMI), and prognostic nutritional index (PNI). This study was performed to estimate the prognostic value of these factors in predicting survival and platinum resistance in ovarian cancer (OC), especially according to stage. Kaplan-Meier and multivariate analyses were performed to plot the survival curve and determine the independent prognostic factors. Additionally, the area under the receiver operating characteristic curve (AUC) was used to predict platinum resistance and prognosis by comparing the predictive ability of PNI and cancer antigen (CA)-125. In all patients, decreased PNI was significantly associated with platinum resistance and poor overall survival (OS) and progression-free survival (PFS). Regarding tumor stage, decreased PNI was significantly associated with poor PFS and OS only in stage III OC. Furthermore, the PNI also showed a significantly higher AUC value than CA-125 for predicting mortality and platinum resistance in all OC patients, but not in stage III patients. In conclusion, decreased PNI is a powerful predictor of a poor prognosis in OC, and especially for stage III cases.