Based on the notion that hypomorphic germline genetic variants are linked to autoimmune diseases, we reasoned that novel targets for cancer immunotherapy might be identified through germline variants associated with greater T-cell infiltration into tumors. Here, we report that while investigating germline polymorphisms associated with a tumor immune gene signature, we identified PKCδ as a candidate. Genetic deletion of PKCδ in mice resulted in improved endogenous antitumor immunity and increased efficacy of anti-PD-L1. Single-cell RNA sequencing revealed myeloid cell expression of Prkcd, and PKCδ deletion caused a shift in macrophage gene expression from an M2-like to an M1-like phenotype. Conditional deletion of PKCδ in myeloid cells recapitulated improved tumor control that was augmented further with anti-PD-L1. Analysis of clinical samples confirmed an association between PRKCD variants and M1/M2 phenotype, as well as between a PKCδ KO-like gene signature and clinical benefit from anti-PD-1. Our results identify PKCδ as a candidate therapeutic target that modulates myeloid cell states.