Abstract Therapies that enhance antitumor immunity have altered the natural history of many cancers. Consequently, leveraging nonoverlapping mechanisms to increase immunogenicity of cancer cells remains a priority. Using a novel enzymatic inhibitor of the RNA methyl­transferase METTL3, we demonstrate a global decrease in N6-methyladenosine (m6A) results in double-stranded RNA (dsRNA) formation and a profound cell-intrinsic interferon response. Through unbiased CRISPR screens, we establish dsRNA-sensing and interferon signaling are primary mediators that potentiate T-cell killing of cancer cells following METTL3 inhibition. We show in a range of immunocompetent mouse models that although METTL3 inhibition is equally efficacious to anti–PD-1 therapy, the combination has far greater preclinical activity. Using SPLINTR barcoding, we demonstrate that anti–PD-1 therapy and METTL3 inhibition target distinct malignant clones, and the combination of these therapies overcomes clones insensitive to the single agents. These data provide the mole­cular and preclinical rationale for employing METTL3 inhibitors to promote antitumor immunity in the clinic. Significance: This work demonstrates that METTL3 inhibition stimulates a cell-intrinsic interferon response through dsRNA formation. This immunomodulatory mechanism is distinct from current immunotherapeutic agents and provides the molecular rationale for combination with anti–PD-1 immune-checkpoint blockade to augment antitumor immunity. This article is featured in Selected Articles from This Issue, p. 2109