Background: Immune checkpoint inhibitors (ICIs) are crucial for treating metastatic urothelial carcinoma (mUC) patients, yet current prognostic biomarkers fail to predict ICI efficacy. Previous preclinical models have suggested myeloid-derived suppressor cells (MDSCs) as a potential therapeutic target in bladder cancer. This study aims to evaluate the predictive value of blood MDSCs in mUC patients undergoing pembrolizumab treatment. Methods: Ninety-eight mUC patients who progressed with platinum-based chemotherapy at Osaka Metropolitan University received pembrolizumab. Among them, 30 patients were assessed for blood MDSCs. Peripheral blood samples were collected from healthy individuals (n = 6) and mUC patients (n = 30) before pembrolizumab initiation. The mononuclear cell layer was isolated using density gradient centrifugation for MDSC analysis through flow cytometry. Monocytic and polymorphonuclear MDSCs were defined as CD11b+ HLADR – CD33 high CD14 + CD15 – and CD11b+ HLADR – CD33 low CD14 – CD15 +, respectively. The neutrophil-lymphocyte ratio (NLR) in blood served as a control. Treatment response was assessed via CT images using RECICT v1.1. Progression-free survival (PFS) and overall survival (OS) were calculated using the Kaplan-Meier method, with log-rank test for between-group comparison. Results: First, blood MDSCs were significantly elevated in mUC patients compared to healthy controls (p = 0.0024, Mann Whitney test). Secondly, patients with MDSC^low (≤ median, n = 15) had significantly longer PFS than those with MDSC^high (> median, n = 15) (17.6 vs. 1.9 months, p = 0.0126). However, no significant difference was observed in OS. In contrast, when the NLR cutoff was set at 3.5, no significant association was found for PFS and OS. Conclusions: Blood MDSCs show promise as a predictive marker for pembrolizumab therapy efficacy.