<p>Asparagine-linked glycosylation protein 1 homolog (ALG1), a β-1,4-mannosyltransferase, catalyzes the initial committed step of the N-glycosylation pathway. While its role in congenital glycosylation disorders is well established, the contribution of ALG1 to bladder cancer (BC) progression, particularly its impact on the tumor immune microenvironment, remains poorly understood. In this multi-omics study, we delineated the pathological significance and molecular mechanisms of ALG1 in BC. We demonstrated that ALG1 directly interacted with programmed death-ligand 1 (PDL1) and catalyzed its glycosylation, as confirmed by site-directed mutagenesis and glycosidase inhibitor treatments. This modification stabilized PDL1 by shielding it from ubiquitin-mediated proteasomal degradation, thereby enhancing its protein stability and cell surface expression. Consequently, ALG1-driven PDL1 glycosylation amplified immunosuppressive signaling via sustained PD1/PDL1 engagement, promoting tumor-associated macrophage M2 (TAM-M2) polarization and facilitating tumor immune evasion. These collective effects reshaped the tumor microenvironment toward an immunosuppressive state, enabling immune escape. Both in vitro and in vivo experiments demonstrated that genetic inhibition of ALG1 effectively reversed PDL1-mediated immunosuppression and restored sensitivity to anti-PD1 therapy. In summary, our study identified ALG1 as a critical post-translational regulator of PDL1 stability and immunosuppression in BC, and targeting ALG1-mediated glycosylation represented a promising therapeutic strategy to enhance immunotherapy efficacy and overcome immune evasion in BC patients.</p>