In aggregate, our work establishes that dual FGFR3 and Trp53 alteration initiates high-grade, non–muscle-invasive, autochthonous murine bladder tumors with an intermediate T cell–inflamed phenotype and that erdafitinib cooperates with PD-1 checkpoint blockade to reverse anti–PD-1–induced Treg expansion and to block progression. This evidence concerns the gene FGFR3 and urinary bladder neoplasm.