During the process of cross-presentation, viral or tumor-derived antigens are presented to CD8(+) T cells by Batf3-dependent CD8 alpha(+)/XCR1(+) classical dendritic cells (cDC1s). We designed a functional CRISPR screen for previously unknown regulators of cross-presentation, and identified the BEACH domain-containing protein WDFY4 as essential for cross-presentation of cell-associated antigens by cDC1s in mice. However, WDFY4 was not required for major histocompatibility complex class II presentation, nor for cross-presentation by monocyte-derived dendritic cells. In contrast to Batf3(-/-) mice, Wdfy4(-/-) mice displayed normal lymphoid and nonlymphoid cDC1 populations that produce interleukin-12 and protect against Toxoplasma gondii infection. However, similar to Batf3(-/-) mice, Wdfy4(-/-) mice failed to prime virus-specific CD8(+) T cells in vivo or induce tumor rejection, revealing a critical role for cross-presentation in antiviral and antitumor immunity.