Background: Fibroblast growth factor 3 (FGFR3) is the most common mutation in UTUC and is altered in approximately 75% of tumors. Tumors harboring FGFR3 mutations (FGFR3-M) have a T-cell impaired tumor microenvironment (TME) which may explain the incomplete response to immune checkpoint blockade. We performed scRNA-seq on 8 untreated tumors to further characterize the T-cell immune phenotype of FGFR3-M tumors. Methods: scRNA-seq (10x Genomics platform) was performed on 8 UTUC tissue specimens from 8 different patients who had not received treatment (chemotherapy or immunotherapy) using an established institutional process. We also performed targeted gene sequencing (MSK-IMPACT) on all samples to identify mutational calls. We assessed the phenotype of defined cell clusters and the immune composition of each sample according to known marker gene expression as well as SingleR prediction. We then performed the gene set enrichment analysis over the differentially expressed genes with the Gene Ontology Biologic Process (GO:BP) to identify unique biologic processes and possible functional state of each immune cluster. Results: Among the 8 samples, 4 (50%) had altered FGFR3 (Table). We identified 19 immune cell clusters (8 T-cell clusters) with unique biologic function. Within the CD4 compartment, FGFR3-M was enriched with exhausted/active CD4 cells characterized with Th17 cell differentiation/immune regulatory function (cluster 4) and yet with lower frequency of naive-like CD4 cells possessing alpha-beta T cell activation functions and lower T-cell receptor (TCR) signaling (cluster 2). Regulatory T cells (cluster 5) were less frequently found in FGFR3-M tumors compared to their wild-type counterpart. In the CD8 compartment, FGFR3-M tumors had higher infiltration specifically in cluster 3 which corresponds to a naïve state with lower exhausted/active markers, lower cytotoxic activity, leukocyte apoptotic process, and alpha-beta T cell differentiation regulation. There was also a lower proportion among cluster 9, a mixture of NK and CD8 cytotoxic cells, which is characterized with response to interleukin (IL)-1, tumor necrosis factor (TNF), and NK cell chemotaxis. Additionally, this cluster had high cytotoxic activity and lower exhausted/active markers. Conclusions: FGFR3 mutated patients have a T-cell phenotype with more active/exhausted Th17-like CD4, lower Treg, and more CD8/cytotoxic cells in naïve state with lower response to IL-1 and TNF. scRNA-seq revealed enrichment of different functional states among T-cell compartments which may lead to improved therapeutic decision making in the future.