Conventional type 1 dendritic cells (cDC1s) capture antigens in peripheral tissues and migrate to draining lymph nodes (dLNs) to prime antigen-specific CD8+ T cells. How tumor antigens are processed to activate CD8+ T cell immunity is not well understood. In this work, we show that Ms4a7 is up-regulated in cDC1s after tumor antigen uptake or exposure to exogenous stimuli and is required for their cross-priming ability. Although Ms4a7-/- mice showed normal cDC1 development and turnover, they failed to prime antigen-specific CD8+ T cells following infection or tumor development. In human cancers, MS4A7 was expressed in a subset of cDC1s, preferentially enriched in dLNs, and correlated with patient survival. Our findings suggest a critical role for Ms4a7 in cDC1-mediated cross-presentation and antitumor CD8+ T cell responses.