In virus-host interactions, nucleic acid-directed first lines of defense that allow viral clearance without compromising growth are of paramount importance. Plants use the RNA interference pathway as such a basal antiviral immune system, but additional RNA-based mechanisms of defense also exist. The infectivity of the plant positive strand RNA virus alfalfa mosaic virus (AMV) relies on demethylation of viral RNA by recruitment of the cellular N6-methyladenosine (m6A) demethylase ALKBH9B, but how demethylation of viral RNA promotes AMV replication remains unknown. Here, we show that inactivation of the cytoplasmic YT521-B homology domain (YTH)-containing m6A-binding proteins, ECT2, ECT3, and ECT5 is sufficient to restore AMV infectivity in partially resistant alkbh9b mutants. We also show that the antiviral function of ECT2 is distinct from its previously demonstrated function in promotion of proliferation of primordial cells, because an ECT2 mutant carrying a small deletion in its intrinsically disordered region is partially compromised for antiviral defense, but not for developmental functions. These results indicate that the m6A-YTH axis constitutes a novel branch of basal antiviral immunity in plants.