Background: Rogaratinib (R) is a novel pan-FGFR inhibitor that showed promising efficacy and safety in a Phase I trial in pts with advanced solid tumors, including UC, with FGFR1-3 mRNA overexpression. The Phase Ib/II FORT-2 study (NCT03473756) of R plus atezolizumab (A) in pts with first-line cisplatin-ineligible, FGFR-positive, advanced/metastatic UC previously identified a maximum tolerated dose of R 600 mg twice daily (BID) plus A (1200 mg every 3 weeks). We report updated safety, efficacy, and the recommended Phase II dose (RP2D) for combination therapy from the Phase Ib study. Methods: Pts with cisplatin-ineligible, locally advanced/metastatic UC with FGFR1/3 mRNA overexpression detected by RNA in situ hybridization of archival tissue (RNAscope) received oral R 600 mg BID plus A 1200 mg on day 1 of a 21-day cycle. Archival tissue was examined for programmed cell-death ligand 1 (PD-L1) protein expression levels, FGFR3-activating mutations via a targeted Illumina NGS panel, and FGFR fusions via an Archer fusion plex assay. Primary objectives were safety, tolerability, and determination of the RP2D. Results: 26 pts (enrolled May 25, 2018 to Nov 25, 2020) were treated; 89% were male, median age was 76 years (range 47-85), 58% had an ECOG performance status of 1, and 77% displayed low or absent (negative or non-detectable) PD-L1 expression (combined positive score < 10%). Common treatment-emergent adverse events (TEAEs) included diarrhea (n = 17, 65%; 1 grade [G] 3), hyperphosphatemia (n = 15, 58%; all G1 or 2), and nausea (n = 11, 42%; 1 G3). The most common G3/4 TEAEs were elevated lipase without pancreatitis (n = 5, 19%), elevated amylase (n = 3, 12%), and rash and syncope (n = 2, 8% each). TEAEs led to interruption/reduction/discontinuation of R in 69%/46%/19% of pts. R-related unique TEAEs were hyperphosphatemia in 15 pts (58%) and retinal pigment epithelium detachment in 1 pt (4%). G5 events occurred in 3 pts (12%), unrelated to treatment. 13 of 24 evaluable pts (54%) had an objective response (OR) per RECIST v1.1. The disease control rate was 83%, including 3 pts (13%) with a complete response (CR), 10 (42%) with a partial response (PR), and 7 (29%) with stable disease. Median duration of response was not reached. OR rate was 56% (2 CRs and 7 PRs) in the 16 pts with tumors having low/negative PD-L1 protein and FGFR3 mRNA overexpression without mutation. The RP2D for R+A was 600 mg BID. Conclusions: First-line treatment with the RP2D of R+A achieved favorable clinical efficacy and tolerability in pts with cisplatin-ineligible, metastatic UC characterized by high FGFR1/3 mRNA expression and generally low/negative PD-L1 expression. Encouraging efficacy was observed regardless of PD-L1 expression or FGFR3 mutation status, warranting future investigation. Clinical trial information: NCT03473756