Myeloid-derived suppressor cells (MDSCs) play a crucial role in tumor-associated immune suppression and in impeding immune checkpoint blockade (ICB) therapies; however, effective intervention strategies targeting MDSCs remain limited. In this study, we observed a positive correlation between spleen tyrosine kinase (SYK) activity in immune cells and tumor progression in human cancers. Utilizing a mouse model with a gain-of-function SYKS544Y mutation, we demonstrated that SYK activation promotes tumorigenesis in carcinogen-induced and xenograft models across multiple cancer types, largely through MDSC-dependent mechanisms. Mechanistically, SYK activates the JAK-STAT-CXCR2 signaling axis, enhancing MDSC migration and suppressing antitumor T cell responses. Notably, pharmacological inhibition of SYK not only inhibited tumor growth but also enhanced the therapeutic effect of anti-PD-1 in mouse models and patient-derived tumor organoid models. Our findings highlight the immunoregulatory role of SYK in tumor progression and suggest that targeting SYK represents a promising strategy to remodel the tumor microenvironment and enhance the efficacy of checkpoint blockade immunotherapy.