A substantial fraction of cancers evade immune detection by silencing STING (Stimulator of Interferon Genes)-interferon (IFN) signaling. Therapeutic reactivation of this program via STING agonists, epigenetic or DNA damaging therapies can restore anti-tumor immunity in multiple pre-clinical models. Here we show that adaptive induction of three prime exonuclease 1 (TREX1) restrains STING-dependent nucleic acid sensing in cancer cells via its catalytic function in degrading cytosolic DNA. Cancer cell TREX1 expression is coordinately induced with STING by autocrine IFN and downstream STAT1, preventing signal amplification. TREX1 inactivation in cancer cells thus unleashes STING-IFN signaling, recruiting T and NK (natural killer) cells, sensitizing to NK cell derived IFNγ, and co-operating with PD-1 blockade in multiple mouse tumor models to enhance immunogenicity. Targeting TREX1 may represent a complementary strategy to induce cytosolic DNA and amplify cancer cell STING-IFN signaling, as a means to sensitize tumors to immune checkpoint blockade (ICB) and/or cell therapies.