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Patients with metastatic high-grade serous ovarian carcinoma are often unresponsive to immunotherapies; here we identify salt-inducible kinases (SIKs) as key drivers of immunosuppression. Human T cells in the presence of patient ascites express high levels of SIK and the upstream kinase LKB1, whereas SIK inhibition reprograms human T cells and strongly activates antitumor responses. In syngeneic mice with resistant high-grade serous ovarian carcinoma, genetic ablation and pharmaceutical inhibition of SIK consistently demonstrated therapeutic efficacy and survival advantages, and combination of PD-1 blockade with SIK inhibition further extended survival. We identified a major role of T cell-intrinsic SIK2 and -3 signaling in driving immunosuppression in part by TXNIP induction and LYST suppression. Multi-omics analyses on SIK inhibitor therapy revealed reduced disease progression, increased T cell infiltration with enhanced cytotoxicity and effector cytokine IFN-γ, and a shift from immunosuppressive to immunostimulatory cellular niche. We propose SIK inhibitors as a new immunotherapy.