Cancers are often accompanied by inflammation, which can promote tumor growth, invasion, and metastases. We show that the tumor microenvironment induces the development of a Gr-1(+) conventional dendritic cell (cDC) subpopulation that is functionally defective. Gr-1(+)cDCs differentiated from recruited immediate precursors of cDCs, a process supported by the inflammatory cytokine milieu in tumors. Inhibition of Gr-1(+)cDC differentiation enhanced intratumor expansion of cytotoxic CD8(+) T cells (CTLs), resulting in suppression of tumor growth. Diphtheria toxin treatment of CD11c-diphtheria toxin receptor chimeras revealed the importance of intratumor cDCs in stimulating CTL proliferation in situ. Our study demonstrates a key role of intratumor cDCs in determining antitumor CTL responses and suggests that they may be an appropriate target for tumor immunotherapy.