Lysophosphatidic acid (LPA) is a bioactive lipid enriched in the tumor microenvironment of immunosuppressive malignancies such as ovarian cancer (OvCa). While LPA enhances the tumorigenic attributes of cancer cells, the immunomodulatory activity of this phospholipid messenger remains largely unexplored. Here, we report that LPA operates as a negative regulator of type-I interferon (IFN) responses in OvCa. Ablation of the LPA-generating enzyme autotaxin in OvCa cells reprogrammed the tumor immune microenvironment, extended host survival, and improved the effects of therapies that elicit protective responses driven by type-I IFN. Mechanistically, LPA sensing by dendritic cells triggered PGE2 biosynthesis that suppressed type-I IFN signaling via autocrine EP4 engagement. Moreover, we identified an LPA-controlled, immune-derived gene signature associated with poor responses to combined PARP inhibition and PD-1 blockade in OvCa patients. Controlling LPA production or sensing in tumors may therefore be useful to improve cancer immunotherapies that rely on robust induction of type-I IFN.