Circular RNAs (circRNAs) are prevalent in eukaryotic cells and viral genomes. Mammalian cells possess innate immunity to detect foreign circRNAs, but the molecular basis of self versus foreign identity in circRNA immunity is unknown. Here, we show that N 6-methyladenosine (m 6 A) RNA modification on human circRNAs inhibits innate immunity. Foreign circRNAs are potent adjuvants to induce antigen-specific T cell activation, antibody production, and anti-tumor immunity in vivo , and m 6 A modification abrogates immune gene activation and adjuvant activity. m 6 A reader YTHDF2 sequesters m 6 A-circRNA and is essential for suppression of innate immunity. Unmodified circRNA, but not m 6 A-modified circRNA, directly activates RNA pattern recognition receptor RIG-I in the presence of lysine-63-linked polyubiquitin chain to cause filamentation of the adaptor protein MAVS and activation of the downstream transcription factor IRF3. CircRNA immunity has considerable parallel to prokaryotic DNA restriction modification system that transforms nucleic acid chemical modification into organismal innate immunity.