Elevation in atmospheric CO2 concentration broadly affects plant phenology and physiology, and these effects may alter the performance of plant viruses. The effects of elevated CO2 on the susceptibility of tomato plants to Tomato yellow leaf curl virus (TYLCV) were examined for two successive years in open top chambers (OTC) in the field. We experimentally tested the hypothesis that elevated CO2 would reduce the incidence and severity of TYLCV on tomato by altering plant defence strategies. Our results showed that elevated CO2 decreased TYLCV disease incidence (by 14.6% in 2009 and 11.8% in 2010) and decreased disease severity (by 20.0% in 2009 and 10.4% in 2010). Elevated CO2 also decreased the level of TYLCV coat protein in tomato leaves. Regardless of virus infection, elevated CO2 increased plant height and aboveground biomass. Additionally, elevated CO2 increased the leaf C:N ratio of tomato, but decreased soluble protein content in leaves. Notably, elevated CO2 increased the salicylic acid (SA) level in uninfected and infected plants. In contrast, elevated CO2 reduced jasmonic acid (JA) in uninfected plants while it increased JA and abscisic acid (ABA) in virus-infected plants. Furthermore, combined exogenous SA and JA application enhanced resistance to TYLCV more than application of either SA or JA alone. Our results suggest that the modulated antagonistic relationship between SA and JA under elevated CO2 makes a great contribution to increased tomato resistance to TYLCV, and the predicted increases in tomato productivity may be enhanced by reduced plant virus susceptibility under projected rising CO2 conditions.