Abstract Purpose: Immune checkpoint blockade has shown remarkable efficacy, but in only a minority of patients with cancer, suggesting the need to develop additional treatment strategies. Aberrant glycosylation in tumors, resulting from the dysregulated expression of key enzymes in glycan biosynthesis, modulates the immune response. However, the role of glycan biosynthesis enzymes in antitumor immunity is poorly understood. We aimed to study the immunomodulatory effects of these enzymes. Experimental Design: We integrated transcriptional profiles of treatment-naïve human tumors and functional CRISPR screens to identify glycometabolism genes with immunomodulatory effects. We further validated our findings using in vitro coculture and in vivo syngeneic tumor growth assays. Results: We identified MAN2A1, encoding an enzyme in N-glycan maturation, as a key immunomodulatory gene. Analyses of public immune checkpoint blockade trial data also suggested a synergy between MAN2A1 inhibition and anti–PD-L1 treatment. Loss of Man2a1 in cancer cells increased their sensitivity to T-cell–mediated killing. Man2a1 knockout enhanced response to anti–PD-L1 treatment and facilitated higher cytotoxic T-cell infiltration in tumors under anti–PD-L1 treatment. Furthermore, a pharmacologic inhibitor of MAN2A1, swainsonine, synergized with anti–PD-L1 in syngeneic melanoma and lung cancer models, whereas each treatment alone had little effect. Conclusions: Man2a1 loss renders cancer cells more susceptible to T-cell–mediated killing. Swainsonine synergizes with anti–PD-L1 in suppressing tumor growth. In light of the limited efficacy of anti–PD-L1 and failed phase II clinical trial on swainsonine, our study reveals a potential therapy combining the two to overcome tumor immune evasion. See related commentary by Bhat and Kabelitz, p. 5778