Background: The association between tumor-infiltrating immune cells and the response to platinum-based chemotherapy (cisplatin vs carboplatin) in advanced urothelial cancer is not well characterized. We dissected the immune microenvironment of 3 large UC cohorts to define populations of immune cells associated with survival following carboplatin- or cisplatin-based chemotherapy. Methods: Two advanced UC cohorts – Hospital del Mar (HM; n = 55) and GREEK (n = 62) – treated with platinum-based chemotherapy were profiled using bulk RNA sequencing (RNA-seq) with tumor-infiltrating immune cells inferred using CIBERSORTx. Our findings were validated in a publicly available platinum-treated UC cohort (n = 93; Taber et al. Nat Comm 2020). Additionally, we assessed the impact of tumor-infiltrating immune cells in IMvigor210, an anti-PD-L1 treated cohort. Analysis was adjusted by tumor stage, and treatment regimen (cisplatin, carboplatin, atezolizumab) to determine associations between tumor-immune cell populations and overall survival (OS). Results: In platinum-treated patients, increased lymphoid cell infiltration was associated with improved OS. Specifically, the presence of naive and memory B-cells and CD4+ T cells was associated with prolonged OS (Table). Interestingly, infiltration of memory B-cells was associated with a better prognosis in patients treated with cisplatin (HR = 0.14) and detrimental in patients treated with carboplatin (HR = 249.53; difference p = 0.006). Additionally, macrophages (HR = 3.44; p = 0.027) and resting dendritic cell (HR = 41.47; p = 0.009) infiltration was associated with worse OS in patients treated with platinum therapy alone, but enrichment of pro-inflammatory M1 macrophages was associated with prolonged OS in patients treated with immunotherapy (p < 0.001). Conclusions: Immune cell infiltration is associated with differential outcomes to cisplatin- and carboplatin-based chemotherapy in advanced UC. Our findings suggest better OS in advanced UC treated with cisplatin in tumors enriched with memory B-cells compared to carboplatin treatment.