Background: Pan-immune inflammation value (PIV) is a newly defined biomarker that includes whole cellular components that are indicators of systemic inflammation in complete blood count (CBC), easily accessible and has the potential to reflect both the body's immune response and systemic inflammation status. This study evaluated the pretreatment PIV for its prognostic impact on overall survival (OS) in patients with metastatic castration-resistant prostate cancer (mCRPC) treated with Lutetium-177 (177Lu)-PSMA-617. Methods: The PIV was based on the earliest CBC obtained within 1 month before treatment initiation. Patients were categorized into low and high PIV groups based on the median pretreatment PIV, and the relationship between OS and PIV groups was assessed by multivariable analysis. Results: A total of 43 patients with mCRPC treated with (177Lu)-PSMA-617 were included. The median OS was longer in the low PIV group (15.1 months [95% confidence interval [CI] 10.6–19.5]) than in the high PIV group (4.2 months [95% CI 1.7–6.6]) (p < 0.001). In multivariable analysis, high PIV (hazard ratio [HR]: 4.3, 95% CI 1.194–15.93, p = 0.026) and high Eastern Cooperative Oncology Group performance score (HR: 7.05, 95% CI 1.48–33.46, p = 0.014) were associated with shorter OS. Conclusion: This study showed that pretreatment PIV might be a prognostic factor in patients with mCRPC treated with (177Lu)-PSMA-617.