The cancer-immunity cycle requires cross-presenting type I conventional dendritic cells (cDC1s) that induce T cell-mediated immunity, but therapeutic strategies for enhancing intratumoral cDC1 function are currently inadequate. We found the epigenetic enzyme CARM1 (coactivator-associated arginine methyltransferase 1) to be a selective negative regulator of cancer antigen presentation by cDC1s but not cDC2s. Inactivation of the Carm1 gene promoted cDC1 antigen cross-presentation, activation, and accumulation in tumors, and a CARM1 inhibitor enhanced cDC1-mediated priming of T cells by means of a cancer neoantigen vaccine. CARM1 inhibition increased chromatin accessibility at BATF3-Jun and RelA sites that are critical for cDC1 function and activation. Transforming growth factor-β regulated Carm1 expression, which suggests that CARM1 inactivation enhanced intratumoral cDC1 function without altering cDC1 homeostasis. These studies identify CARM1 as a potential therapeutic target for enhancing the antitumor function of mouse and human cDC1s.