Background: Radical surgery is the mainstay of treatment for muscle-invasive upper tract urothelial cancer (UTUC) and urothelial bladder cancer (UBC). However, even when surgery is combined with cisplatin‐based (neo)adjuvant chemotherapy, recurrence rates are high. Many patients (pts) are ineligible for, do not respond to, or refuse cisplatin-based chemotherapy because of toxicity concerns. Fibroblast growth factor receptor 3 (FGFR3) genetic alterations occur in 21–38% of muscle-invasive UTUC, and in approx. 20% of invasive UBC. Infigratinib (BGJ398) is an ATP-competitive FGFR1–3 selective oral tyrosine kinase inhibitor that has clinical activity and tolerability in pts with advanced/metastatic urothelial carcinoma (UC) harboring susceptible FGFR3 alterations. PROOF 302 was designed to investigate the efficacy and safety of adjuvant infigratinib in pts with high-risk, muscle-invasive UC (85% UTUC) and susceptible FGFR3 alterations. Methods: PROOF 302 is a global, multicenter, randomized, double-blind, placebo-controlled, phase 3 trial (NCT04197986). Eligible pts have high-risk, muscle-invasive UTUC (85%) or UBC (15%) with susceptible FGFR3 alterations (mutations, gene fusions or rearrangements) confirmed by FoundationOne CDx test, and are ≤120 days following nephroureterectomy, distal ureterectomy, or cystectomy. The study protocol was amended to include pts with carcinoma in situ at surgical margins and those who refuse cisplatin-based chemotherapy (in addition to pts allowed to enroll if ineligible for perioperative cisplatin-based chemotherapy). Eligible pts are M0 and have either disease stage ≥ypT2 and/or yN+ after cisplatin-based neoadjuvant chemotherapy or ≥pT2 pN0–2 or pN+ (UTUC); and ≥pT3 or pN+ (UBC) if not treated with cisplatin-based neoadjuvant chemotherapy. Pts are randomized 1:1 to receive oral infigratinib 125 mg or placebo QD on days 1–21 of a 28-day cycle for up to 52 weeks (13 cycles) or until recurrence, unacceptable toxicity, or death. Primary endpoint: centrally reviewed disease-free survival (DFS) analyzed via stratified log-rank test and Cox model (hazard ratio). Secondary endpoints: investigator-reviewed DFS, metastasis-free survival, overall survival, safety/tolerability. Exploratory endpoints: quality of life, pharmacokinetics, determination of the prevalence of genomic alterations and correlation with features of UC, genomic and proteomic assessment of tumor tissue and cell-free DNA samples from baseline and at recurrence to identify the predictive/prognostic value of biomarkers. PROOF 302 is actively enrolling as of Sep 2021. A second Data Monitoring Committee meeting is planned for Dec 2021. The last pt is expected to complete treatment in 2024. Clinical trial information: NCT04197986.