Background: In this paper, it was aimed to evaluate the biomarker potential as well as the effect of the prognostic nutritional index (PNI), which is calculated using the albumin level reflecting nutritional status and lymphocyte count reflecting immune status, in determining the prognosis of metastatic castration-sensitive prostate cancer (mCSPC). Methods: This retrospective observational study included the complete data of 108 patients with mCPSC who were treated for at least three months between January 1, 2010, and June 1, 2021. The relationship between cancer specific survival (CSS), overall survival (OS), progression free survival (PFS) and PNI was evaluated. Kaplan-Meier method for OS, PFS, and CSS, as well as univariate and multivariate Cox regression models were used in statistical analyses. Results: The median age of 108 patients included in the study was 68.54 (61.05-74.19) years. While 71.3% (n = 77) of the patient population were high-volume according to CHAARTED, 52.8% (n = 57) were high-risk based on LATITUDE. 49.75 was determined as the best cut-off point for the PNI. OS (months) was found to be significantly lower in patients with low PNI (median: 34.93, 95% CI: 21.52–48.34) compared to patients with high PNI (median: 65.60, 95% CI: 39.36–91.83) (p=0.016). Patients with high PNI (median: 48.20, 95% CI: 34.66–61.73) had significantly better CSS (months) than patients with low PNI (median: 27.86, 95% CI: 24.16–31.57) (p=0.001). Conclusions: PNI calculated at the time of diagnosis strongly predicts OS and CSS but not PFS in patients with mCSPC.