There is intense interest in the advancement of RNAs as rationally designed therapeutic agents, especially in oncology, where a major focus is to use RNAs to stimulate pattern recognition receptors to leverage innate immune responses. However, the inability to selectively deliver therapeutic RNAs within target cells after intravenous administration now hinders the development of this type of treatment for cancer and other disorders. Here, we found that a tumor-targeting, cell-penetrating, and RNA binding monoclonal antibody, TMAB3, can form stable, noncovalent antibody/RNA complexes of a discrete size that mediate highly specific and functional delivery of RNAs into tumors. Using 3p-hpRNA, an agonist of the pattern recognition receptor retinoic acid-inducible gene-I (RIG-I), we observed robust antitumor efficacy of systemically administered TMAB3/3p-hpRNA complexes in mouse models of pancreatic cancer, medulloblastoma, and melanoma. In the KPC syngeneic, orthotopic pancreatic cancer model in immunocompetent mice, treatment with TMAB3/3p-hpRNA tripled animal survival, decreased tumor growth, and specifically targeted malignant cells, with a 1500-fold difference in RNA delivery into tumor cells versus nonmalignant cells within the tumor mass. Single-cell RNA sequencing (scRNA-seq) and flow cytometry demonstrated that TMAB3/3p-hpRNA treatment elicited a potent antitumoral immune response characterized by RIG-I activation and increased infiltration and activity of cytotoxic T cells. These studies established that TMAB3/RNA complexes can deliver RNA payloads specifically to hard-to-treat tumor cells to achieve antitumor efficacy, providing an antibody-based platform to advance the study of RNA therapies for the treatment of patients with cancer.