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Conventional dendritic cells (cDCs) are a critical immune population, composed of multiple subsets, and responsible for controlling adaptive immunity and tolerance. Although migratory type 1 cDCs (CD103 + cDC1s in mice) are necessary to mount CD8 + T cell-mediated anti-tumor immunity, whether and how tumors modulate CD103 + cDC1 function remain understudied. Signal Transducer and Activator of Transcription 3 (STAT3) mediates the intracellular signaling of tumor-associated immunosuppressive cytokines, such as interleukin (IL)-10; thus, we hypothesized that STAT3 restrained anti-tumor immune responses elicited by CD103 + cDC1s. Herein, we show that in vitro-derived STAT3-deficient ( Stat3 ∆/∆ ) CD103 + cDC1s are refractory to the inhibitory effects of IL-10 on Toll-like receptor 3 (TLR3) agonist-induced maturation responses. In a tumor vaccination approach, we found Stat3 ∆/∆ CD103 + cDC1s restrained mammary gland tumor growth and increased mouse survival more effectively than STAT3-sufficient CD103 + cDC1s. In addition, vaccination with Stat3 ∆/∆ CD103 + cDC1s elicited increased amounts of tumor antigen-specific CD8 + T cells and IFN-γ + CD4 + T cells in tumors and tumor-draining lymph nodes versus phosphate-buffered saline (PBS)-treated animals. Furthermore, IL-10 receptor-deficient CD103 + cDC1s controlled tumor growth to a similar degree as Stat3 ∆/∆ CD103 + cDC1s. Taken together, our data reveal an inhibitory role for STAT3 in CD103 + cDC1 maturation and regulation of anti-tumor immunity. Our results also suggest IL-10 is a key factor eliciting immunosuppressive STAT3 signaling in CD103 + cDC1s in breast cancer. Thus, inhibition of STAT3 in cDC1s may provide an important strategy to improve their efficacy in tumor vaccination approaches and cDC1-mediated control of anti-tumor immunity.