The paucity of costimulation at the tumor site compromises the ability of tumor-specific T cells to eliminate the tumor. Here, we show that bi-specific oligonucleotide aptamer conjugates can deliver costimulatory ligands to tumor cells in situ and enhance antitumor immunity. In poorly immunogenic subcutaneously implanted tumor and lung metastasis models, systemic delivery of an agonistic 4-1BB aptamer ligand conjugated to a prostate specific membrane antigen (PSMA)-binding tumor-targeting aptamer led to inhibition of tumor growth, was more effective than, and synergized with, vaccination, and exhibited a superior therapeutic index compared to costimulation with 4-1BB antibodies. Tumor inhibition was dependent on homing to PSMA-expressing tumor cells and 4-1BB costimulation. Aptamer targeted costimulation is a broadly applicable and clinically feasible approach to enhance the costimulatory environment of disseminated tumor lesions. This study suggests that potentiating naturally occurring antitumor immunity via tumor-targeted costimulation could be an effective approach to elicit protective immunity to control tumor progression in cancer patients.