Background: The aim of this study was to explore the prognostic value of the lung immune prognostic index (LIPI) in patients with metastatic hormone-sensitive prostate cancer (mHSPC) and metastatic castration-resistant prostate cancer (mCRPC). Methods: We retrospectively analyzed data of 502 mHSPC patients primarily treated with maximal androgen blockade and 158 mCRPC patients receiving abiraterone. All cases were classified into the LIPI-Good, LIPI-Intermediate, and LIPI-Poor groups based on their LIPI score calculated by the derived neutrophil-to-lymphocyte ratio and lactate dehydrogenase. The potential of LIPI in predicting mCRPC-free survival (CFS), PSA response, PSA-progression-free survival (PSA-PFS), and overall survival (OS) were analyzed. A propensity score matching (PSM) methodology was performed to balance the baseline factors of different groups. Results: In the mHSPC cohort, patients of the LIPI-Good (mCFS: 25.7 mo; mOS: 93.3 mo), LIPI-Intermediate (mCFS: 14.8 mo; mOS: 51.9 mo) and LIPI-Poor group (mCFS: 6.8 mo; mOS: 18.5 mo) had sequentially worse clinical outcomes (P<0.001 for all pairwise comparisons). The results remain consistent after PSM. Multivariate Cox regression further confirmed that LIPI was an independent predictor of survival outcomes. Subgroup analysis verified that LIPI was associated with unfavorable prognosis in all subgroups except for cases with visceral metastases or receiving abiraterone/docetaxel. As for patients with CRPC receiving abiraterone, LIPI was also an indicator of poor prognosis. Specifically, cases in the LIPI-Good, LIPI-Intermediate and LIPI-Poor group had a ladder-shaped worse PSA response (71.4% [50/70] vs. 56.5% [39/69] vs. 36.8% [7/12], P=0.015), PSA-PFS (14.9- vs. 9.3 vs. 3.1 mo, P<0.001) and OS (14.6- vs. 32.3- vs. 53.4 mo, P<0.001). The results were robust even after PSM. Multivariate Cox regression confirmed that LIPI was an independent prognosticator of abiraterone. Conclusions: This study demonstrated that baseline LIPI was a significant prognostic biomarker for both mHSPC and mCRPC patients and could potentially facilitate risk classification and treatment-decision making in clinical practice.