The cellular source of positive signals that reinvigorate T cells within the tumor microenvironment (TME) for the therapeutic efficacy of programmed death-1 (PD-1)/programmed death-ligand 1 (PD-L1) blockade has not been clearly defined. We now show that Batf3-lineage dendritic cells (DCs) are essential in this process. Flow cytometric analysis, gene-targeted mice, and blocking antibody studies revealed that 4-1BBL is a major positive co-stimulatory signal provided by these DCs within the TME that translates to CD8⁺ T cell functional reinvigoration and tumor regression. Immunofluorescence and spatial transcriptomics on human tumor samples revealed clustering of Batf3⁺ DCs and CD8⁺ T cells, which correlates with anti-PD-1 efficacy. In addition, proximity to Batf3⁺ DCs within the TME is associated with CD8⁺ T cell transcriptional states linked to anti-PD-1 response. Our results demonstrate that Batf3⁺ DCs within the TME are critical for PD-1/PD-L1 blockade efficacy and indicate a major role for the 4-1BB/4-1BB ligand (4-1BBL) axis during this process.