536 Background: Microsatellite instability (MSI) and deficient mismatch repair (dMMR) are associated with sensitivity to immune checkpoint blockade (ICB) in urothelial carcinoma (UC). MSI status may only partially explain this sensitivity, and the molecular features that distinguish MSI-high (MSI-H) UC are not well described. Methods: Retrospective review of UC patients having undergone targeted exome sequencing of up to 505 genes was performed. Using the previously validated MSIsensor score, we identified MSI-H (score ≥10) and MSI stable (MSS) (score <3) UC tumors. Tumor mutational burden (TMB) was quantified. Gene alterations enriched in MSI-H UC were evaluated using the false discovery method with q<0.05 considered statistically significant. Mutational process signatures (MPS) were characterized using COSMIC v3.3. Survival analysis was performed using the Kaplan-Meier method with Cox proportional-hazards to determine progression-free (PFS) and overall survival (OS) based on molecular features. Results: Targeted exome sequencing data were available for 3,811 UC specimens representing 2,608 patients, of which 471 carried a diagnosis of upper tract UC. Of the total, 60 (1.6%) MSI-H tumors from 55 (2.1%) patients were identified. Germline testing (n=52) revealed dMMR mutations in 24 cases (46.2%) with MSH2 (59.6%) being most common. Median MSIsensor score was 19.8 (IQR 15.5, 27) and median TMB was 52.7 mut/Mb (IQR 39.7, 63.2). Notable gene alterations enriched in MSI-H tumors are summarized in Table 1. Dominant MPS in MSI-H UC included dMMR (85.0%), APOBEC (8.3%), and Aging (6.7%). MSI-H tumors clustered with dMMR mutational signatures SBS6 and SBS44. A total of 21 MSI-H and 160 MSS patients having received ICB for metastatic disease were identified for survival analysis. Median follow-up was 18.4 months (IQR 6.6, 58.5). MSI-H UC demonstrated superior PFS (HR 0.34, 95% CI 0.22-0.55) and OS (HR 0.48, 95% CI 0.28-0.80) following ICB. There was no association between TMB or MPS and survival following ICB in MSI-H UC. In MSS UC, TMB ≥10 mut/mB was associated with improved PFS (HR 0.56, 95% CI 0.40-0.79) and OS (HR 0.71, 95% CI 0.50-1.0). Conclusions: Targeted sequencing reveals distinct genomic features of MSI-H UC. The findings from this relatively large clinical cohort of MSI-H UC patients affirm previous associations between MSI status, TMB, and response to ICB in metastatic UC. [Table: see text]