Although RNA cytosine-5 methylation (m⁵C) is an important post-transcriptional regulatory mechanism, its contribution to plant antiviral immunity remains unclear. In this study, we identified Thiamine thiazole synthase 2 (TaTHI2) as a host mRNA target of the wheat m⁵C methyltransferase TaNSUN2 during infection by Chinese wheat mosaic virus (CWMV), a soil-borne virus that poses a major threat to wheat production. TaNSUN2 contributes to the m⁵C modification of TaTHI2 transcripts, enhancing mRNA stability and sustaining TaTHI2 accumulation. The disruption of a key m⁵C site markedly reduced methylation, weakened TaNSUN2–RNA binding, and accelerated transcript decay, leading to the compromised production of reactive oxygen species (ROS) and increased viral infection. Mechanistically, the TaNSUN2-dependent m⁵C modification stabilized TaTHI2 mRNA, thereby promoting ROS-mediated antiviral defense. Collectively, our results establish the m⁵C modification of TaTHI2 mRNA as a critical post-transcriptional control point in CWMV resistance and highlight TaNSUN2-dependent RNA methylation as an integral component of host antiviral immunity.