Infection of plants by multiple viruses is common in nature. Cucumber mosaic virus (CMV) and Turnip crinkle virus (TCV) belong to different families, but Arabidopsis thaliana and Nicotiana benthamiana are commonly shared hosts for both viruses. In this study, we found that TCV provides effective resistance to infection by CMV in Arabidopsis plants co-infected by both viruses, and this antagonistic effect is much weaker when the two viruses are inoculated into different leaves of the same plant. However, similar antagonism is not observed in N. benthamiana plants. We further demonstrate that disrupting the RNA silencing-mediated defense of the Arabidopsis host does not affect this antagonism, but capsid protein (CP or p38)-defective mutant TCV loses the ability to repress CMV, suggesting that TCV CP plays an important role in the antagonistic effect of TCV toward CMV in Arabidopsis plants co-infected with both viruses. •TCV provided effective resistance to infection by CMV in Arabidopsis plants co-infected by both viruses.•This antagonistic effect cannot be reproduced in N. benthamiana plants.•Disrupting the RNA silencing-mediated defense of the Arabidopsis host does not affect this effect of TCV toward CMV.•The capsid protein (CP or p38)-defective mutant TCV loses the ability to repress CMV.