Cells in the tumor microenvironment (TME) communicate via membrane‐bound and secreted proteins, which are mostly glycosylated. Altered glycomes of malignant tumors influence behaviors of stromal cells. In this study, we showed that the loss of core‐1 β1,3‐galactosyltransferase (C1GALT1)‐mediated O‐glycosylation suppressed tumor growth in syngeneic head and neck cancer mouse models. O‐glycan truncation in tumor cells promoted the M1 polarization of macrophages, enhanced T‐cell‐mediated cytotoxicity, and reduced interleukin‐6 (IL‐6) levels in the secretome. Proteasomal degradation of IL‐6 was controlled by the O‐glycan at threonine 166. Both IL‐6/IL‐6R blockade and O‐glycan truncation in tumor cells induced similar pro‐inflammatory phenotypes in macrophages and cytotoxic T lymphocytes (CTLs). The combination of the O‐glycosylation inhibitor itraconazole and anti‐programmed cell death protein 1 (anti‐PD‐1) antibody effectively suppressed tumor growth in vivo. Collectively, our findings demonstrate that O‐glycosylation in tumor cells governs their crosstalk with macrophages and CTLs. Thus, targeting O‐glycosylation successfully reshapes the TME and consequently enhances the efficacy of anti‐PD‐1 therapy. This study found that O‐glycan truncation induced proteasomal degradation of interleukin‐6 to suppress tumor growth by promoting M1 macrophage polarization and enhancing T cell‐mediated cytotoxicity in head and neck cancer in vitro and in vivo. Combining an O‐glycosylation inhibitor with anti‐programmed cell death protein 1 antibody effectively suppressed tumor growth, emphasizing the role of O‐glycosylation in modulating the tumor microenvironment.