Abstract The accumulation of high levels of adenosine in tumors activates A2A and A2B receptors on immune cells and inhibits their ability to suppress tumor growth. Deletion of adenosine A2A receptors (A2AARs) has been reported to activate antitumor T cells, stimulate dendritic cell (DC) function, and inhibit angiogenesis. In this study, we evaluated the effects of intermittent intratumor injection of a nonselective adenosine receptor antagonist, aminophylline (AMO; theophylline ethylenediamine) and, for the first time to our knowledge, a selective A2BAR antagonist, ATL801. AMO and ATL801 slowed the growth of MB49 bladder and 4T1 breast tumors in syngeneic mice and reduced by 85% metastasizes of breast cancer cells from mammary fat to lung. Based on experiments with A2AAR−/− or adenosine A2B receptor−/− mice, the effect of AMO injection was unexpectedly attributed to A2BAR and not to A2AAR blockade. AMO and ATL801 significantly increased tumor levels of IFN-γ and the IFN-inducible chemokine CXCL10, which is a ligand for CXCR3. This was associated with an increase in activated tumor-infiltrating CXCR3+ T cells and a decrease in endothelial cell precursors within tumors. Tumor growth inhibition by AMO or ATL801 was eliminated in CXCR3−/− mice and RAG1−/− mice that lack mature T cells. In RAG1−/− mice, A2BAR deletion enhanced CD86 expression on CD11b− DCs. Bone marrow chimera experiments demonstrated that CXCR3 and A2BAR expression on bone marrow cells is required for the antitumor effects of AMO. The data suggest that blockade of A2BARs enhances DC activation and CXCR3-dependent antitumor responses.