Dendritic cells (DCs) are recognized as the most potent antigen-presenting cells, capable of priming both naïve and memory T cells. Thus, tumor-resident DCs (tumor-associated DCs: TADCs) play a crucial role in the immune response against tumors. However, TADCs are also well known as a "double-edged sword" because an immunosuppressive environment, such as a tumor microenvironment, maintains the immature and tolerogenic properties of TADCs, resulting in the deterioration of the tumor. Therefore, it is essential to maintain and enhance the anti-tumoral activity of TADCs to aid tumor elimination. This study demonstrated the potential for tumor growth inhibition of Aureobasidium pullulan-derived β-glucan (AP-BG). Administration of AP-BG dramatically limited the development of different types of tumor cell lines transplanted into mice. Examination of the tumor-infiltrating leukocytes revealed that AP-BG caused high expression of co-stimulatory molecules on TADCs and enhanced the production of cytolytic granules as well as pro-inflammatory cytokines by the tumor-resident T cells. Furthermore, the syngeneic mixed lymphoid reaction assay and popliteal lymph node assay showed the significant ability of AP-BG to improve DCs' antigen-specific priming of T cells in vitro and in vivo. Taken together, β-glucan might be an immune-potentiating adjuvant for cancer treatment. This highly widely-used reagent will initiate a new way to activate DC-targeted cancer immune therapy.